An immortal magus does some long complicated process and gains a Might score - thus becoming immune to warping and old age.
When they get their Might, do they get Qualities and powers? What about when they improve their Might?
As they still have their arts, spells and enchantments, I would say no - but I can also see the argument for it. After all, the magus has been transformed into something beyond human...
Although speaking of transformed, I'd totally allow the use of Transformation to acquire Qualities and Inferiorities for an immortal magus. But yeah, TMRE came out before RoP:M, so there's no chance of you being granted something that doesn't exist.