a) the mirror problem: either retract it in advance for LCD use, or use a fixed partially reflecting mirror as in the Olympus E10/E20
the focal plane shutter problem:
Ahha, I'm surprised I didn't think of the mirror being in the way. Of COURSE the mirror is in the way. Thank you.
I wouldn't mind a button to flip up the mirror. I'd prefer keeping a fully reflective mirror to preserve all the light going to the viewfinder, and losing the viewfinder when the mirror was up for LCD previewing.
A purely electronic shutter makes sense; I'm surprised they don't do that already. (Maybe it helps keep dust off the sensor?)