This is really murky. I see what HoH:Societates says about creating artificial objects with Creo and the same level of success being required. I'm not sure I've ever followed it. Call it an unspoken house rule that Creo crafting is "easier" or whatever you'd like.
Conjuration of the Seaworthy CogI'll note that the core rule book doesn't set an ease factor for the Int+Finesse roll. But another book, Hermetic Projects uses a low Ease Factor for (Projects, 65) . I suppose because it is an exact copy of a template ship used during the design of the spell? On the other hand, using Rego magic would require an EF of 15+ depending on the size of the boat based on the text for the spell The Hands of Phereclus (Projects, 63).
I'm totally A-OK with the idea that Creo magic is easier (lower finesse rolls, uses Int), were I to house rule creating things with Rego Craft magic making it easier to do, I would also drastically lower the EF for Creo/vis.
One concession I will make, on further thought, is that the base difficulty for all the Finesse rolls (not counting the extra time modifier) would be the same as for the entire structure at once; a masterpiece cathedral requires masterpiece foundations and components. Doing it piecemeal would only reduce the time constraints modifier.
You're being more restrictive than I'd be here - remember that as an alternative to having a Craft Score of 1 for a basic level of familiarity, something that commonly occurs in an area where you have an Area Lore score of 1 also counts, which would almost certainly include swords and houses in covenants (although probably not so much cathedrals). In fact, you could probably argue for a +3 familiarity bonus for a sword, on the basis that (non lab-rat) magi are likely to encounter them on a daily basis
But that's required for the Momentary version anyway. The point is that different pieces of the whole might be manipulated at different times if the whole is the target.
A related but different view: changing to D: Concentration adds a magnitude. It's essentially been established that adding magnitudes can add a bonus to the Finesse roll via the spell and comments on Conjuring the Mystic Tower. The whole controversy might be reduced by adding magnitudes for this purpose instead of changing to D: Concentration.
When I did D: Concentration I did with with painting, something where the whole thing wouldn't fall apart if only partly done.
I'm not going to do an exhaustive search of every spell that creates something. I'll stipulate to your point that every other spell discusses a needed finesse roll. That being the case, it isn't explicit stated in the description that a finesse roll is unnecessary or to use your term, eliminated. And without that explicit instruction, I would then hold that the need for the finesse roll would be there as described on page 77.
Most magi won't be able to make the roll. CtMT creates a tower that is a single stone. Medieval folks trying to do this would take years, maybe even 10 or more. That's a target of, what, 6+3+12=21? A right big waste of vis for most.
(And why bother with 3 magnitudes for complexity if there's a Finesse roll that accounts for this?)
First, I'm not convinced that it would require any modifications for time that a Rego crafter would have to overcome. Remember for Rego crafting, you set the difficulty of what it would take for a mundane craftsman and add +3. That's the base difficulty, and then you add based on time factors to create the product. Were that the case, to include increases in the EF for time, and for the very reason you bring up, no magus would be able to create anything artificial with any degree of success. I might even, and this would be HR territory, to be sure, avoid the +3 above what a mundane craftsman would need.
I'd probably set the difficulty as average, and it's almost assured with an Int +3, Finesse +3, especially if Confidence is used.
Following up: The rules for crafting and creating art through magic yield very high target numbers, to the point of placing it out of reach for all except the most dedicated specialists. (Creo vs Rego for this purpose has a difference of 3 for targeting, btw. That's it.) Oh, and it's a stress roll, so magi get to botch things like milking a cow. They also cannot benefit from workshop bonuses, etc. I suspect this was deliberate, to justify why magi need all those peasants and stuff. HoH:S is explicit about the consequences of crafting rules on artistically inclined magi. Covenants and A&A, not explicit.
So, by my reading of RAW, you set the difficulty for a task, as if it were being undertaken by a craftsman. Then you add 3. That's your base EF to meet. Now, if your doing it by Rego methods, you also add modifiers for time.
I think that's a pretty creative reading. The rules are clear about the length of time for Rego: mundane+3+time. The rules are clear that creating stuff has the same target as using Rego.
C'mon, it's no more creative than reading that CtMT has eliminated the need for a Finesse roll because it isn't actually listed in the spell description.
I'll point out that Conjuring the Mystical Tower has +3 for elaborate design, while Conjuration of the Seaworthy Cog has +3 for intricacy but specifically lists the an EF for Int+Finesse.
It's dangerous, I've found, to assume that Ars Magica is consistent in its writing of spells - or even some of its spell guidelines. (PeAq spells violate the second sentence Aquam section of spells).
CtMT makes an artificial thing (elaborately carved tower), not a natural thing, so there's a Finesse roll to determine how good it is, but you'll always get a useful tower unless you Botch (per pg 77 rules on Creo magic).
