I actually did a similar trip last Dec.... used 125K miles to fly F on BA SEA-AMS with a 3-day stopover at LHR.

So yes to #1, a stopover is allowed (in one direction I think). Another option if to do an open-jaw ticket... handy if you plan to hop on a train to another part of Europe or take one of their discount carriers.

#2, yes.

#3, Depends on the dates. Also, some award seats open up closer to flight time. To many people's surprise, my return from AMS was on Dec 22nd... normally a blackout date for many airline awards, but BA had them available. In fact, I was the ONLY person in F on the return... entire international first class 747 BA cabin to myself. No wonder they opened up seats! I'm sure during the summer seats are VERY TOUGH to get.

Good luck... BA offers an excellent F product!

P.S. If two of you are traveling, try getting row 1. Seats are close enough together, you both get nearly forward facing windows, and besides people using the closet at the beginning and end of flight, it's rather private.