1. No, you can use the premier line.

2. SAN-IAD will definitely arrive at either C or D. Impossible to tell you which one it'll be, could honestly be C2 or D29, and anything in between. The IAD-AMS flight, from my experiences walking thru IAD during the European departure rush, will more than likely depart from somewhere in between C1 and C14. But that's not 100% guaranteed. Either way, you should have plenty of time unless the flight is delayed. And every time I've taken SAN-IAD (6 or 7 times), it's been early in IAD, often by quite a bit. You should be fine.

3. Been to AMS twice but never used a lounge, so can't help you there. I have heard the lounges for UA passengers in AMS are pretty awful.

4. Any UA club you want.

Have a great trip!