Specifically, I'm talking about the Elixir and the Living Ghost.
In case of the elixir, what's the point? Wouldn't a mystery initiation for Transformed Human be better? Something like the Amazon Living Goddess, except the Gift is retained.
In any case, my real question is whether or not both are affected by acclimation, in which case, both are effectively limited by Might 50 and I'm not sure that being immortal is worth it, especially since the Code may or may not apply to you. Might 50 is pretty much nothing and you're gonna be dead and somebody harvesting your organs in short order! Furthermore, for elixir, even to get to Might 50 is a chore.
Am I missing anything here?
(I guess that only Criamons and Merinitae can be truly immortal and still be functional magi?)
For my Mythic Pennies...
It's been a while since I used TMRE, but it helps to think of Mystery virtues as benefits of advancing throught the respective mystery cults rather than an ends in and of themselves. Ideally then, a character who is initiated into "Great Elixer" or "Living Ghost" is one who has already devoted to Alchemy or Spirit Magic respectively and would be doing such stuff anyway. Incidentally, the same goes for the Exoteric Mystery Cults as well - bjornaer, criamon and merinita are among my favourite houses, but the characters only involve themselves in their houses' inner mysteries if they are in line with that character's magical interests (usually they're not).
Personally, I'm of the impression that Transformed Human is overrated. Yes, it offers a more immediate power-boost, but characters initiating start very weak - Might 0 and no qualities. Normally improvements can be made using the advancement mechanics in RoP:M, but such a character, being relatively weak (and especially if young) will derive less benefit from the "Immortal Magi" appendix in the TMRE supplement.
I think you've answered your question rather well.
"How do the Mysteries and RoP:Magic mesh ?"
I don't remember the exact mechanics, so can't comment on Transformed Human vs. Great Elixir/Living Ghost. I would say that TMRE seems designed more with PCs in mind whereas RoPM more for the SG, so I'd be careful of using the ROPM mechanics and my natural tendency is to use the TMRE ones instead.
As to your "real question" - yes, it does seem the "immortal magi" will be subject to acclimation and thus limited to Might 50. And yes, this means they'll have very low MR - but then again, they'll have fairly low MR even with Might 100, which puts them up there with the primeordials of the Magic Realm... Whatever Might you give them (unless you exceed 100), competent Hermetic magi would be able to surpass it if they really want to.
I'd suggest instead to consider what role these immortal magi serve in the saga, and house rule things accordingly. Perhaps they're Secret Masters, in which case just finding them is the difficulty ? Or maybe they are Gods on Earth, in which case boosting their MR might be suitable ? Do what works for you.
Personally, I'd prefer boosting up all MR (as in: every creature's MR) to about (Might x 3), and may fudge the formulas to get their Might to be below 50. I'll leave the acclimation thing as-is; Might 50 is the most powerful you can get this side of the Magic Realm. If they want more power - they'll have to join the Tangential Magi in the Twilight Void...
I also don't think the two mesh together well at all.
Mysteries introduces immortals as the end result of a long process of mystical and magical development. The unchanging nature of the immortals is both flavorful and game balancing.
RoP:M is different. Acclimation makes things unnecessarily difficult for Mysteries immortals. To compensate new mechanics are introduced for advancement but these are, imo, fairly flavorless. In general the RoP:M rules remind me of pure points system games like Hero much more than they do traditional and flavorful AM products. I tend to ignore almost everything from RoP:M.
I think this is a good approach for a GM book. Gives you tools with a lot of latitude to work with. For a PC, now... well, players just shouldn't read RoPM