#1 Yes, assuming you cut it back beyond any parallel section it might have.
#2 The angle of the cone leading into the choke has much less to do with the amount of choke produced than does the muzzle ID. Muzzle ID, compared to bore ID, is the most important factor. Yes, you certainly will open up the choke with a straight reamer.
#3 Not necessarily. I understand Winchester has done just that, usind a reamer with something like a 20' radius and coming through the muzzle farther for a more open choke and less far for a tighter one. In these cases the actual muzzle ID varies, as with all chokes but there is no parallel section.
