This was my original perception, too. It seems, though, that the consensus is that any kind of supernatural trapping requires giving up the pure mortal bonus, even if, mechanically, it's nothing more than a supernatural aspect and normal invokes and compels.
Ah ok now i understand, that requires the possibility of effects through Aspect compels and the like, which personally, i think is not how it works.
You don't get trappings through aspects. Aspects only provide legitimation for certain trappings.
As i already mentioned before, I think invokes, compels and declarations do not constitute effects a character generates. They constitute changes the
player makes to the game world based on certain aspects of the character. From a character perspective they are not changes or effects at all, they don't happen... they just are.
So if you actually want the character to produce effects, you need a power, and as soon as you have a power you're not a pure mortal anymore. But even then, you cannot produce effects through aspect invokes or compels. So yeah, supernatural trappings of any kind, since they require a power, make you loose your pure mortal bonus.
Anyways, that has nothing to do with being able to take on powers later. That's possible for any and all characters, pure mortal or not.