As I recall, we can codify spells with masteries, and I thought those spells can get new masteries (after some thought i don't know if that was ever said, just that it works that way for new spells, and I guessed it extends). However, codifing spells is hard, and the version with masteries is even harder.
So my guess would be that recursively mastered spells are possible, just takes a lot of time, because the codification step is a large barrier, and the Colleges aren't that old.
That could explain how the Nekharans did some real shit. Though on the other hand, those spells don't really match what you'd expect from such a process, and people like the Druids should have them too.
So either there's some metaphysical distinction between the main part of the spell and the mastery of which there can only be one...
Or I'm poking the abstraction too hard.
Could you clarify that?