1. Per diem at most companies is based on "time away from base" - IE the per diem clock starts when you show up for work on day one and runs continuously until you leave for home at the end of your trip on the last day. Some companies pay different rates of per diem for domestic vs international trips.

2. Dunno, since I haven't flown different derivatives. That said, I'd imagine most companies utilize "common procedures" that ensure that you will remain in the flight envelop for the most restrictive of the different aircraft. IE if you tail strike at 18 degrees in airplane A and 14 degrees in airplane B, you fly both aircraft with the 14 degree takeoff rotation limit until off the ground.

3. Ditto. Never flown the MD-11.