I think some kind of Profession: Administration skill is necessary for covering the ins and outs of less obvious logistics, economics and accounting.
That said, I really do agree in the broad sense that 5E has FAR too many skills, and atomizing professions just takes things too far. The other point is that Ars Magica is a simulationist game, and not a heroic adventuring game like say, Mutants and Masterminds (which ironically has a similar problem with the Expertise skill.) D&D and Exalted have seen massive flame wars of just how granular craft and profession skills should be. This is clearly not unique to Ars Magica.
Just to be clear, I lumped management and accounting into the same skill, in the spirit of a broad skills that you promote, so we are only talking about one competency. I too thought of Artes Liberales, and if that had not already has such a broad application that many consider it unbalanced, I would have been content with that.
What I see, however, are certain recurring game situations where an actual specialist should bring a benefit. A random PC with common and broad skills should simply not allowed to be the expert in something entirely outside their background.
One is covenant loyalty, which is improved by a skilled autocrat. It is unbalanced and bland if the resident magister who teaches letters to prospective apprentices can suddenly do the autocrat's job expertly.
Similarly, the story-teller or poet can often have a critical role to make a good impression at a faerie court. If any PC with positive Com/Pre and fluency in the language is a good storyteller, many such stories are just dull.
But, of course, if these situations are not significant in the saga, the skills are redundant.
Not myth. It is canon. Servants and labourers are the broad categories covenfolk who are paid less than anybody else, have no skill worth mentioning, and who can be recruited implicitly to fill necessary numbers [Cov].
I would argue that this is in fact most of what the large estate administrator does. In contrast, while the farmer is also planning, that's only a small amount of their time, and they're doing so entirely within the grounds of existing abilities. Profession: farmer should include basic farm planning, but I don't think it would cover the type of administration skill that isn't domain specific.
For me personal what Ars Magica put into Profession Scribe is actual Calligraphy and so should be in Craft. Calligraphy isn't just about knowing the meaning of the writing but instead about knowing different style how you can write something, how to handle the pen or brush without leaving inkblot and also how fast you can write without mess up so that you have to start over.
That's possible, but still tricky. Which Ability in your saga then pertains to the organization, use and trade of books? Which to text organization, indices, canon tables, the very important Latin shorthands, and where to put which illustrations, initials and tables?
I can imagine a Profession: LIbrarian alongside a Craft: Scribe.
That is not really an issue. We already assume that with Craft: Smith, the craftsman is skilled in the organisation and trade of his goods. No need for scribe to different.
Profession: Scribe is off because the principal application in canon is the crafting of copies. You are of course right that there are other applications for which no mechanics is defined, and I would not advocate introducing a new profession skill to cover that.
To me the label has no importance. A scribe is a scribe whether we label them professionals or craftsmen, and I see no practical difference between Profession: and Craft:. (And I struggle with Aristotle's distinction between techne and phronesis too. Distinction ok, but why is it important?)
Oh. That explains it fully I think. I believe Covenants put a ban on speeding up copying with magic. There is a canon spell which explicitly is not faster than a mundane scribe. I don't have time to search for a more specific and universal ban. However, the ban, I believe, is necessary for game balance. By making scribe a profession, instead of a craft, we have escaped the haunt of craft magic.
Librairies can be duplicated in the blink of an eye already. Learn Twinning the Tome (HoH:TL p. 101-102) for +10 lab total bonus, then research a new version with +2 group and +1 size to level 65, and you are creating a copy of an entire covenant's library for the sweet price of 13 vis.
Interesting. I was not aware of that spell. However, Twinning the Tome is already group target, presumably to target a group of pages. Thus your calculations do not make sense, but I wonder if you could extend with a different argument.
Anyway. This is a ritual, so you cannot cast it in the blink of an eye. And luckily, not using the principles of craft magic, it makes no case for achieving the result with Rego magics and raw materials.
Maybe, but a group of groups is not entirely the same thing as a larger group. There is structure which arguably could call for an extra complexity magnitude. But I suppose the counter-arguments would be exhausted at L65 even so.
It takes a bit to invent it in game though. The canon spell is boosted, and of no use to rank-and-file Hermeticists. Inventing it from scratch need a quite decent lab total with two requisites. Nothing for the non-specialist. It takes enough advancement that it is unlikely to be a game breaker. That's somewhat reassuring.