So anybody in possession of a stripped lower when the ban went into effect could do anything they wanted with it, within the constraints of the NFA?
A stripped lower registered pre-ban was exempt, so, yes, you could build the rifle as desired within NFA constraints.
How did the shift in supply/demand affect prices on things like flash suppressors? I'm assuming there were fewer people purchasing & wondering how manufacturers reacted.
Prices on flash suppressors remained stable, partly because demand went way down. It was prices on pre-ban rifles (and any pre-ban stripped lowers) that shot straight up. People suddenly wanted the forbidden fruit and demand far exceeded supply.