On #4, reasonably sure it's just not worth it for them to compete with United on this hub-to-hub route.

I've taken the AA flight IAD-BOS recently. For me it was nice as I was able to upgrade to first class with only one segment, and I was the only person in F. The flight seemed lightly loaded, but then again, the last time I took the DL shuttle IAD-BOS, there were only 6 pax on the plane.

The Admiral's Club at DCA is nice and the staff is friendly. I've never made it past the work cubicles to see what the actual lounge area is like.