A) I've only been to the club in T-4 but it's nice. Your flight will almost certainly depart from T-4 too and getting between the two terminals isn't easy. Can't help much more on if you can use other clubs. Don't know that one as I haven't tried.

B) You will have SkyClub access at DCA. Your whole itinerary should display to the agent when you check-in, and he/she should see the JFK-SFO leg. If you get any flack when you check-in at the club, present you D1 boarding pass for JFK-SFO.

C) 5D is a good window seat. Any of the odd row numbers have better window views than the even rows because the seats are closer to the window. This also gives you a little more privacy from the aisle. I personally like 1A and 1D because the seats have a little bit bigger footwell than the other does.