Of course, this has nothing to do with my original remark, which was about how the existence of Great Elixir immortals proves that it is possible to be a fully-supernatural being which is competely sustainable in the long-term, yet it retains some significant advantageous feature of human nature and hence is not as limited as other kinds of supernatural, once-human crreatures from the same realm. Since they are eerily similar to half-Become mages who did not transform the mind, by analogy it strongly suggests that a variant of Becoming may exist by which you do not need to make your Mind as static as a vanilla faerie and may be sustained indefinitely.
This is nothing to do with being divided between two Realms, since Transforming the Spirit (necessary for full immortality) completely remakes the Gift as a Faerie power, anyway. And about "has one foot in the Magical Realm, and one in the Faerie Realm with regard to its magic - which is not generally how AM works", sorry, but this is a rather dumb comment: any mage with Faerie Magic has its Gift working just the way you decry.
Which feature are you referring to, beside them being alive, material and not off beyond the lunar sphere? They still have the same handicap to learning as other immortals do.
I was referring to the fact that, according to RoP:M, all other types of human being transformed to Magic supernatural beings do not have such flexible and broad-based powers: theirs are generally bound to one only Form, and a set of Formulaic-like tricks, or the ability to use spont-like effects in a strict Focus, narrow than a single Form. This includes mages that become Magic beings by other means, but not mages who used the specialized immortality methods developed by Mystery cults (Noble's Parma: I do not remember which category a mage that would become an immortal supernatural artist would fall in).
If Hermetic research could find a loophole to such an apparently basic feature of Magic begins as being tied to one Form, I reason out that it ought to allow to develop a variant of Becoming that does not require a vanilla faerie's static mind to be sustained indefinitely. In Faerie terms, human heritage would allow to be a story which is self-aware (half-Become mages would all be highly cognizant faerie) that can freely rewrite their own details (they can make unrestricted use of spontaneous magic) even if big changes (e.g. learning a new formulaic spell) still requires magic (ie the change rituals) to be fixed.
As the disadvantages of full Becoming go, it is the inability to use spontaneous magic that sees IMO the heaviest burden, which all but the most honest faerie groupies would try to escape, and which TMRE immortals do not face (inability to use fatigue is bad enough, another burden that Elixir and Living Ghost immortals do not have, even if I keep thinking that Become mages ought to be allowed to use Faerie Might in place of fatigue to fuel magic). The other problems with learning seem balanced.
Maybe I am more aware of the issue, since I play a Merinita spont specialist. He's highly interested in immortality (we twice met various kinds of immortal mages in our chronicle, once a recurring NPC who become immortal by tasting the fruits of Eden, the other an alchemical immortal) but honestly, with all due respect for his Quendalon House-mates, he would rather sell his soul to a demon than see his whole Hermetic development destroyed by using full Becoming. He would rather pursue the Great Elixir, thank you. I reason out that other Merinita mages should interested in keep using unrestricteed spont magic after Becoming, since Merinita magic is already somewhat oriented to flexible magic.
Problem is, this cherry picking is not balanced, as it allows you to become fully immortal without any difficulty to learn anything more, contrary to all other immortality methods.
I fully acknowledge the difficulty. OTOH, full Becoming is just as greately unbalanced, with the impossibilty to use Fatigue and the crippling ofspont magic. Maybe somewhat in the middle would be necessary ?
Hmm. To have full immortality (with which I believe you mean "immunity to both aging and warping") it seems a character would require both Transformation of the Body and Transformation of the Spirit. So from your words it seems that you consider Transformation of the Mind something that strongly inhibits one's ability to learn - and thus a handicap that balances the otherwise unbalanced boons provided by the other two Transformations.
Personally, I think you are under-estimating the usefulness of Transformation of the Mind wheninconjunctionwiththeothertwotransformations. It does not slow down a character's advancement in those abilities he has: it actually accelerates that advancement by 50% or more (it's probably better than having "affinity with everything")! True, it does make learning completely new things harder, but how hard?
For formulaic/ritual spells, not hard at all (see "the Empowered Faerie" on p.96). For abilities, much more only if the magus already has a lot of experience in other areas. In practice, if a character spends a little time learning the basics of stuff he might need before attempting the transformation, this probably means an inability to learn new languages, area and organization lores - paired with the requirement to keep all abilities but arcane ones at lowish levels if one wants to master new spells. This is something of a handicap, but nothing huge. Finally, Transformation of the Mind inhibits a magus' ability to spont stuff very different from what he already knows - but note that 1) there are ways to get around this stuff (e.g. thaumaturgy) 2) sponting is already crippled if you have Transformed the Body, and in the long run a character should have virtually all the formulaics he'll need.
So, in a nutshell, I do not think that "full" Becoming is more "balanced" than Transformation of the Body and Spirit alone. Whether it is balanced against other "immortality" effects is a different, and far trickier question (I am mildly inclined to say it is, but it's not difficult to defend the opposite position).