A magus will be on the brink of final twilight, though. A single botch and he is gone for good, having accumulated more than his share of Warping from being in rock form.
Xavi
Even better. Finally a use for Creo Vim spells...
It's a discrete form because Muto cannot kill a man. It can, however, turn him into something which makes it easier to kill him. Likewise, Creo cannot kill a man, but it can be used to create situations which will kill him - Creo Ignem is pretty damned good for making humans die, after all. For the ultimate instant kill spell, just go with ReMe and Call to Slumber - works even on giants as Mentem doesn't require size modifiers for the individual, and whilst they're asleep just slit their throats.
Keys are better than daggers at opening doors, but you can still use a dagger to do so - merely threaten someone until they open the door. Likewise, a key is a poor weapon unless you force it into someone's throat at which point the relatively swift death of a slit throat may look like a mercy. Creative use of magic and the world in which it exists is part of what makes Ars Magica fun. Changing spell levels to reflect not magical power or difficulty but rather something-my-players-might-try-therefore-it-should-be-hard is a poor way to build a game, I feel.
OK. Last attempt. I'm clearly not going to convince you, but I'll make my case for the general readership.
In your saga, you can do whatever you want, but this is explicitly prohibited in the MuCo guidelines in ArM5, to paraphrase, Muto Corpus does not affect the target's mind.
So, in your saga, if I take a pint of water away from a magus that has used Transform to Water spell is he damaged? If so, that's not RAW. You can do whatever you want in your saga, but I think the original question was about RAW. Ignoring a portion of the rules because you don't understand the reasoning for them is fine, but you're no longer playing RAW. A man who turns to mist, water, dirt, stone, glass, etc. becomes a man-shaped example of that substance. When the spell ends, he reverts to human form, regardless of what you did to the substance during the spell's duration. When a man-shaped body of water splashes to the ground and becomes a puddle, the person does not return as a puddle of human goo. To me, there is no difference between a person being in a puddle on the ground after transforming to water and being a pile of dirt. Both result in the person coming back unharmed.
I don't know the reasoning, but I would hazard a guess that it's not legacy. David's attempt to more clearly define the Forms is likely at work here. If one allows a large chunk taken away from a transformed person to result in damage to that person, Muto has gone into the territory of Perdo. Perdo is used to hurt and destroy; Muto is for transformation. Transformations don't do damage, unless it is incidental. A fish needs water to live; a pile of dirt in the shape of a man does not have to stay in the shape of a man to continue its existence.
There are dozens of examples of transformed people dying when you shatter their transformed form. I would be extremely surprised (and break my suspension of disbelief) if the glass dude reformed from the scattered remains of the ex-glass statue I have been beating with a hammer for Diameter duration.
Xavi
I believe A&A pp. 24-25 can be employed to make arguments in both directions.
In Xavi's case, a man (quality of flesh, blood, man) gets an added quality of glass. His shape isn't changed by the spell. We then have a glass man. It is the nature of glass to break into pieces when whacked properly with a hammer. This shatters it, and makes it effectively lose the form of a man. At this point, the pile of shards has the quality of flesh, blood, glass. The spell expires, the added quality of glass disappears. The quality of man has not been changed by magic, but I'd say it is nevertheless gone at this point. The man is dead.
This is metaphysics, though. I'm very sure an argument can be fairly made in the other direction. 
In general I agree with you. I would not have a problem with Muto turning a person into a statue even with the expectation of knocking the head off that statue afterwards, just as one could knock the head off of a sleeping person. However, the original example of turning a person into dust, with the intention of blowing it away, looks very much like a death spell to me. Add a Rego requisite to knock the dust apart and I think most would agree that it IS a death spell.
This is not to knock the spell concept. It's a much more stylish and interesting death spell than it would be if it was simply "PeCo - you die", which is all the guidelines require. Nonetheless, it is firmly in Perdo territory in my opinion.
It comes down to whether one privileges the spirit of the Technique (I and the rest of us have used Form instead here, incorrectly) over the numerical guidelines, or vice versa. The issue only seems to arise with Muto, which is defined so broadly that one can imagine it encompassing not only Perdo by also Creo (I transform the dirt/air into whatever I want to create) and Rego (I carve out precision machinery by transforming parts into air and blowing them away). I say that if Muto can substitute for the other Techniques, at least it must be harder to do than it would be by the intended method.
Indeed, and I would allow Muto's "key" to kill but not as efficiently as Perdo's "dagger". Hence the extra magnitudes.
Yes. In much the same as as if you cut off the foot of a magus under the effect of Shape of the Woodland Prowler and burn it, the spell does not become permanent and the spell target is crippled. I note that the RAW, at least in the Core which is all I have with me, does not prohibit this. Presumably this is thus not the case in your saga?
With MuCo(Te), you can turn a man into a statue; you can also turn him into a lump of natural looking rock. These require the same guideline, but have very different effects, and which occurs is a matter of choice for the magus designing the spell. (For added fun, you can only change an individual to an individual - thus turning a man into a diamond statue is nearly impossible without adding size modifiers as well.[1])It is true that the RAW say that the tranformation does not affect (change, rather) the mind, but then immediately contradicts that by going on about the transformation affecting the minds and how the weak willed will forget they are human.
Finally, at what point has the italicised sentence had anything to do with what I've said? The spell ends; the puddle returns to being a man. The problem only occurs when half the puddle has been drunk by a passing horse and another third has been scooped up and poured over a rose bush on the other side of the road. Likewise, the pile of sand will return to being a man unless someone with a shovel scatters it to the four winds. The transformation does no damage; the subsequent damage persists after the spell ends.
[1] For extra added fun, changing a random selection of rocks in a wall to diamond has some seriously counter intuitive results. The rocks shrink, but the wall is not damaged because Muto turns it into a wall which has holes in it which still stands. Move the diamonds, however, and you're left wondering at what point the metaphysics gets confused.
Irt would be funny to turn a man into dressed stone for Moon duration, then leave in the pile of stone being used to build the castle and where they are about to place the foundation stones. Maybe the stories about men beuing buried alive to grant the tower of London's stability have some foundation (pun intended) in them 
Cheers,
Xavi
You, sir, are an evil and twisted man, with a mind whose convolutions would put the very devil to his knees. As such, I wish to subscribe to your newsletter.
For added fun, allow the stone to continue to perceive his surroundings
Ahh....I see why we disagree so much. The guideline is turn a person to rock, not turn a person into a statue. I disagree that you can turn someone into a statue using the same guideline. I think that guideline allows you to turn the target into rock, dirt, glass, etc., which is man-shaped, but they are just that substance, which happens to be in man-shape. If that man-shape no longer happens to be the shape of the substance when the spell ends, there is no negative consequence. Stone isn't hurt by breaking it, it just becomes two pieces of stone. Stone has to be pulverized to be damaged. Statutes, however, are damaged by taking chunks of them away, just like animals are.
I think the guidelines are for turning people into rock, not turning them into statutes or windows or other things that can be easily destroyed or damaged. If a player wants to turn a person into a statue and not a rock shaped like a man, I'd add some magnitudes and force a Finesse roll.
A rock with human shape is a statue... A human body transformed in rock maintain his form... then...
Somebody could discover the real nature for thats statues, but should be soebody very keen or somebody very learned in sculpture. Since the rock in that you transfoms the target it's decided in the spell design, a rarely rock to create statues (like could be slate) the the way to awareness should be easier.
A question, another way to become somebody inrock it's create a magicall ill that become in rock (A&A guidlelines suggesting hermeticla diseases), chould that magical ill spell requisites?
John, if smash the head of a man turned to statue with a hammer, and beak his arms so that they fall to the ground, will the man be unharmed when the Diameter duration "turn man into rock" ends? Just curious here....
Xavi
I really can't agree with this. When I think of using Muto to turn a person to stone I think of the classic Gorgon's Gaze petrification spell. This is what Hermetic Magic was designed to simulate. It wasn't intended to only allow a magus to turn things into amorphous pieces of rock, barring some difficult random roll to make them look human.
A pox on whoever invented the Finesse skill!
No, this is not the case. A statue is "A three-dimensional form or likeness sculpted, modeled, carved, or cast in material such as stone, clay, wood, or bronze." If there is nothing done to the rock by human hands, it's just a rock that looks like a human shape. There are lots of examples of rock features that have vaguely human shapes that have been caused by erosion or other natural forces. They are not statues; they are rocks.
Likewise, there is nothing in the rules that says a body transformed to rock maintains its form. That's your personal preference in your saga, which is fine, it's just not required by the rules. I, personally, would not go that way, as I've stated above, and will state below. My position isn't supported by the rules either, it's just a personal preference.
It is my personal opinion that you could go either way on this. The rules are clear that a person transformed into water is not harmed if something happens to part of his watery body, but if too much is separated, he cannot revert to human form. I think this is because moving around parts of water doesn't harm the water. Just like moving around pieces of rock or dirt doesn't make rock or dirt any less rocky or dirty. Moving around or destroying a part of a statue makes it a worse statue. It's my opinion that you cannot turn a person into a statue using Hermetic magic. You turn him into rock, which takes the shape of a human and may look like a statue if you have a good Finesse roll. Because of this belief, I would apply the Transformation to Water rule. However, if Hermetic magic can turn a person into a statue in your saga, I would say the person comes back as a bloody stump just like a wounded animal comes back as a wounded person.
Just because Hermetic magic was designed to simulate this, doesn't mean that it can do it perfectly.
You can do whatever you want in your saga, but I would suggest making a Muto Corpus spell that turned a person in a statue, rather than into a pile of rocks, harder and require a Finesse roll. I understand your frustration with the Finesse Ability, but I think the other examples in HoH: Societates reinforce my position. In our saga, we allow bonuses to apply to Finesse rolls based on the perfection of a model used in creating an item through magic. There could be no more perfect model for a statue of a man than the man himself. I could easily see adding +6 to the Finesse roll to make him into a statue. We also give a +6 to all Creo magic when creating items; therefore, the unmodified HoH: Societates chart is only used for Rego magic.
John, if your magi transforms a bald man with a leg in a wolf and his obese wife and their two sons and three daughters, the male wolf would be without a leg and had less fur in the head, the female wolf would be obese and the cubs should be three female and two males, that in less words it's declared by the rules too in the Muto Corpus guidelines.
Naturaly the rock it's raw rock, but the difference should be only by an expert eye. A peasant could think that this human shaped rocks are statues, just like many miracley statues or dwarf/giant works that exist in the folklore.
That's close to my take on it too. The spell doesn't need to change the shape of the target in any way. It's just changing all of the Co into Te. I think you should get a statue that's uncannily realistic, better than any sculptor could make, because it's an exact copy of the target. Of course, if idealized images are in vogue everyone will hate your work.
I appreciate John's suggestions on making Finesse rolls easier, but I see no need to involve the skill in any way with this sort of spell. The difficulty is handled by your skill in Mu, Co, and Te. Arts should include the ability to do useful and delicate work, not just brute force effects. The latter makes Hermetic Magic boring and D&Dish. I'm open to his suggestion that the levels could be too low; I haven't had much personal experience with these effects in-game but certainly some level guidelines may need adjustment.
Can you turn a man into a statue, or must it be a shapeless rock? Shape of the Woodland Prowler turns you into a natural wolf, not a man-shaped one. It is the same thing for Stance of the Patient Tree. Both of these examples match the shapeless rock theory.
Now, could you do a partial transformation and end up with a man-shaped wolf or a man-shaped tree? There are many level 5 spells that give the target an animal quality and I believe giving them multiple qualities (fur, head, tail) shouldn't be any harder than the full transformation. Creating a fake werewolf should be as easy as a natural wolf.
Therefore, it should be as easy to MuCo into a shapeless rock than as into a statue, and Finesse should only be used if you try to make the statue look like someone else.
Scattering like Light (HoH:S p63) destroys the original object permanently, which seems to imply that scattered dust will not return to human form. But this may be because the species are absorbed when they strike a non-reflective surface.
What about wounds? Animals and trees can be wounded, earth water and air cannot. If you damage a statue, will it turn into a wound when it reverts to human? That is still open in my mind.
If we turn a man into a salt statue and dip it in water to shrink it, he should not have had his skin melted nor should he become smaller when the spell ends. If you cast Transform to Water and boil yourself away, it should not damage you upon revert. All these point to no damage when shapeless.
Even if the man reforms from his scattered dust, he may still die. Move the dust in an iron maiden. Blow him up in the air and watch him fall. Put his hand in a box to maim him. Disperse him out of his armor to make him defenseless. None of these require Perdo to kill, just an extra magnitude for Rego.
Call to Slumber is a level 10 death sentence, turn to fish is a level 35 death sentence, why can't a level 40 turn to dust be one?
Specifically, the spell is described as a way of metal coating things and thus strongly argues that the bits return to being bits, just evenly distributed over the surface of the non-reflective surface. Casting the Corpus equivalent on a man should then result in the bits returning to flesh, but spread out over such a massive surface that noone would notice anything other than perhaps a slight red tint on white clothes.