Q1: quite a regular occurrence. No need to take the seat out to find out, just feel around underneath. If you feel a couple of flappy bits, then yes the diaphragm is bust. Yes, that would cause your discomfort. Exmoor trim make replacements, or search around on the D1 forum for alternatives.
Q2: No. D2 seats will not fit. Argument about if they are better or not is a moot point. You want D2 seats, buy a D2.:p

