First of all thanks to all of you for the commitment into this little project of mine. I'm not replying back as soon as I would like mostly due to the fact that after reading you, giving it some thoughts and digging into rulebooks, I came back to read some of you already saying what I was going to reply (edit: it even happened twice while I was typing this).
Summarizing things, yes I will stick to the rules even for this NPC. There are two reasons for this. The main one is that if the players are constrained to them, I think it's fair to do the same. I really hate bringing to the table the "this is at it is because I am the Storyteller and you are not". Yes, I will stick out of Divine and Infernal realms because they add contexts that I would like to avoid: I don't want the character goals to be stuff about eternal damnation or saving. I had been thinking about the Faerie Realm (maybe giving this NPC Faerie Magic), which could be handy to get some fun parameters for his Gruagach spells, but I also find myself frowning upon that. When I fantasize about this character, I figure his motivations to be his own alone, and that self focus (let's call it that) is only that only the Magic Realm gives.
I find myself more open to grant him access to Mystery Virtues thought; he would be extremely old; by the time of the foundation of the Order he would be 240 years old, which probably would make him one of the most powerful sorcerers alive in Mythic Europe (and that brings some interesting things to think about: was he known? Did Trianoma visited him? Did he even meet Bonisagus? Maybe he did, but he had his reasons not to join the Order? Maybe he sent an apprentice there, something went sideways and that's why he started to hate the Order and kept himself away from it?... and this can go on and on and add some extra unexpected background to the character). And as Mystery Cults knowledge must come from somewhere, I feel that the guy could had been the source of some Mystery Virtues which later were learned and used by Mystery Cults.
Back to the mechanics, it seems clear that any virtue which gets rids of aging rolls grants 1 warping point per year and just because of that the age limit would be 275 years, until reaching the Warping Score of 10 and getting turned into a troll for good. So no way they can work.
So when I read Callen suggesting Transformed Being I start considering that: getting Might, and so becoming a Magical Being, would get rid of these mundane concerns like aging and warping, wouldn't they?
But there are some issues there as well...
If the virtue comes from a Give Blessing spell, and the spell is made constant (or cast repeatedly with long duration), it would probably also give warping to the character, so once the spell ends or is interrupted then, well, we get the troll again. And there is also the issue of Gruagachan not being able to cast blessings upon themselves, as Tellus and Darkwing pointed...
But the virtue could come from Initiation (more on that and on how I would take any virtue happily later), and with it we would get rid of both the aging and warping problems. The first issue that came to my mind would be that his Might would either be quite low (the base would be 0, or up to 10 if he gets inferiorities and spent the earned qualities in Might), and so an easy target for any Might Stripping spell. OK then, good news is that he wouldn't be quite humpered to keep learning because of he having Might. Bad news would be that an apprentice spont-ing a PeVi Might stripping over him would put an end to his long life quite easily (or would it? Would External Soul work then?). So what I'm thinking now is how to raise his Might, if not constantly (which would be a problem, and probably non required for a being that spent centuries on hideouts), for these times when he rampages the Order every other century.
The Grant Might spell on The Great Elixir section of The Mysteries suggest this could work: it's a CrVi constant effect that grants might equals to half (effect level - 20). Of course it ends saying that it's a unique mystery spell (but as I said, I wouldn't mind using it and claiming that this Mystery Cult actually got this from Dav'nalleous (or whoever this ends up being) himself) and that it may not be cast as a spell, which maybe I could override saying that though it doesn't work with an Hermetic's CrVi it does with a Gruagach's GiBl. Deducing the base is tricky because of the constant duration modifiers, but if the base is (level - 20) / 2 I guess it could be (level - 15) / 2 or even (level - 10) / 2... or going wild, just level / 2. Well, let's say it's (level - N) / 2. Let's do some estimations... if he ends up having 10500 xps, his Techniques could easily come in the range 14-16 (or up to 20 with affinity, +3 if Puissant) and Forms in the range 32-37 (or up to 45 with Affinity, +3 if Puissant), so with N = 15 he could probably put a GiBl spell of level around 65 to toy with the final might, considering spell duration, the N value and such... And there again lies the issue of Gruagachan not being able to cast GvBl upon themselves (and maybe try to get rid of that limit through Breakthroughs? Or a Virtue?).
Whatever can be solved through virtues can happen: the guy is going to create his own Mystery Cult and devote time to invent initiations. Probably he will get Potent Sorcery from Rival Magic, maybe Cthonic Magic (the extra durations and ranges would be quite appreciated, as doubling the lower art), and such. So whatever solutions coming from Mistery, Magic or Supernatural virtues could get in easily.