My thoughts, as the guy who tries to nerf power all the time:
My suggestion would be to make the Master Blacksmith and Master Mason powers actually cost 3-4 might, which restricts how often he can use them but not restricting how powerful they are. With a single activation of Mason, he can craft ten cubic paces of stones - more than enough for a den. How many times a day does he really need to activate that? Slowly build a full sized house? It'll take maybe two or three rounds. And I think having him build an Armillary Sphere over the course of a season instead of over a day is not a huge problem.
I had a Verditius in a one-shot game who had a a Blacksmithing craft-spell and a spell to conjure a pile of iron bars, and they could fix a LOT of problems pretty casually. I think if you make him only able to use the power 2-4 times a day it's fine.
You can redirect some of those mastery points to making the Unseen Servant have better control, allowing him to steal/sneak away with books and carefully turn pages without damaging them.
I believe there's a 'constant power' modifier you can add to personal powers that costs a magnitude.
Also: If he is designed to become a familiar, then he is going to be able to ignore XP costs pretty soon.
Just a note, your Might is actually 14 (10 - Size) and you still have 4 points of Magic Qualities to spend. You start with a total equal to your Might, which for your racoon is 14, and have only spent 10 of them.
The thing is a familiar who starts off powerful can get stupid powerful based on the speed of the game. My familiar Bastet started on the same power level (Summer Animal of Virtue) as Micrometon and now has over 50 years of experience added.
Since they lack the XP penalty, Familiars have a much easier time of achieving 'Transformation' (RoP:M, p.52) which can get very out of hand if you focus on it. My Familiar has only done the XP: 10 version three times so not that bad (improving Might by +2 and taking another Improved Characteristics), but one of the other players familiars has spend north of 210 points.
Even without 'Transformation' a Familiar will get many seasons to study what they want (any time their Magus is busy studying or such which they cannot contribute to). That lack of XP penalty will mean they can end up with all sorts of skills. If they know how to read then they can spend any season their Magus is reading doing so as well. Of course that is assuming they have access to books, which comes from ether a Master willing to get one for them, charming the librarians, or being very sneaky.
I believe you get Qualities and Inferiorities based on the original might, not the size-modified might. The example character on p35 of RoPM certainly followed that trend.
Hummm, you are right. The text is at the bottom of the first column on p.32 "(This does not change the number of Magic Qualities the character receives, as described below.)".
However his Might is still 14.
EDIT: And I should have known better, since looking at my familiar she only had the 10 points of free Magic Qualities. I did miss one of her Transformations, so she actually has four of the 10XP version, with the one missed adding Improved Damage (Claws).
As a note, Micrometon familiar-to-be will play quite differently that Micrometon NPCs. One is looking forward companionship and trading potential immortality for a trusted, well-educated friend is not an issue, whereas the other one is too independent to consider that an option and will only resort to such a deal if it is the only way to survive.
As @raccoonmask said, using a bunch of inferiorities to lower might score to allow a magical being to keep learning is powergaming, considering the amount of qualities you get back in return. Sure, it is trading 5 magic resistance but the benefit far outweight the downside. Hence, okay for an NPC.
Regarding Transformation, I have a slightly different take on them than @InfinityzeN experienced: it still takes a master or at least some excellent source of knowledge to get one, and in my Saga, that will be more the limiting factor than the XP progression. Practice with a source quality of 8 (the highest suggested in the core rule book) make is useless except for the lowest might magical character (even for NPC Micrometon with reduce might), adventure is barely better, so only teaching remains. At least mages have books, but magical animals...
You are not taking into account that Familiars do not reduce their XP gain by their Might. Even with only Practice a Familiar could trigger the 10 XP level with about two seasons.