I can't find any level 10 CrHe guideline. Where does it come from?
No, for your purpose this might not be a cop out indeed! Just for @ezzelino's objection, it would have been.
The mastiff might on its own not be intelligent enough to conduct a defensive fight against multiple opponents attacking a spell-casting magus over minutes - but certainly could be set upon an opponent and fight him until death. There are other spirits who can do this - like those of dead legionnaires and shield grogs.
I got the ReHe guideline from ArM, p. 138. I originally wrote CrHe because it's early and I'm fallible (sigh). My book's quite old, and that may have been errata'ed. Also, most, but not all, of the example spells do require concentration for the plant matter to do something useful, which is unhelpfully contradictory.
Thanks. This is apparently the guideline, which the MoH p.44 effect The Movement of a Man enchanted into Gwidion's Riding Tree talisman uses.
It shows the obvious: that "move with purpose and intelligence, without requiring your constant control" does not mean to imbue a plant with an Intelligence Characteristic or A&A p.31 reason and intellect.
Again, I think you are overconstraining yourself. Which is fine if you like the challenge! But otherwise unnecessary.
Remember that guidelines are not meant to be exhaustive. In general, Hermetic magic is meant to be universal, and with a few exceptions, the limits of magic and a sense of balance and aesthetics are all that should limit you. Controlling a substance "in an unnatural fashion" can be extremely open-ended: for example, vanilla ReIg can make a fire into a humanoid shape that performs a dance! Similarly, you do not need anything exceptional, save possibly for extra magnitudes for complexity, to make something that appropriately reacts to the environment (the corebook examples of Imaginem are a good source of inspiration); a Mentem requisite is necessary and sufficient if, and only if, you require "rationality" on par with a human (even if a very stupid one). By the examples in the core book, you do not need some Hermetic breakthrough, freak magical accident or integration of ancient magic to do it.
Let me also add that animating a blade to count coins or talk through some sort of sign language might be hard, but to perform its "natural" function of fighting is certainly at least as "appropriate" as making a vine to entangle a victim, and would not in my view require a Mentem requisite -- unless perhaps if you tried to have it feint, maneuver for terrain, and perform similar feats that are beyond the cunning of animals (well I think they are - animals often surprise me with how smart they can be).
For those who like closure, this is what our troupe settled on.
In the near term, my maga will be making and using this spell:
Graceful Flight of the Unfettered Brand (ReTe 25)
R: Voice, D: Sun, T: Ind
Causes a sword, knife, or similar bladed weapon to take flight and hover near the caster. At any time during the duration, the caster can cause the weapon to move as she sees fit. For as long as she maintains concentration, she can use her Perception + Finesse (plus the blade's normal statistics and quality bonuses, with a Strength of 0) to attack and defend with the weapon at a distance. The blade counts as being engaged with an opponent, preventing him from simply running past it to attack the caster. When the caster ceases concentrating, the weapon reverts to floating at her side.
(Base 3, +2 metal, +2 Voice, +2 Sun)
In the long run, she may be creating an enchanted item that generally follows the guidelines given by ezzelino above. Our troupe is fine with allowing a sword to attack in a straight-forward way that doesn't use skill, but rather agility and speed. They're more of a high-fantasy loving group than I am, and they come at it from an aesthetic and mythic point of view more than from the game mechanics and balance end of things.
However, I am really quite taken with the image of my maga training the blade to attack, heel, and so forth in the covenant yard. I really like the idea of her playing fetch with it, and of it being able to act on its own. So, I'm quite likely to go with the expensive route and actually use the spirit of a well-trained guard dog to give it some autonomy.
That sounds like an awesome and enjoyable idea. A flying sword that acts like a puppy. "Who's a good sword..."