1.) United's only nonstop service from DCA at this time is to their ORD hub. Pre-9/11, they flew nonstop to MIA (for South American connections) but that has not yet returned. And IIRC United has never flown DCA-DEN nonstop.
2.) Remember, the Congressional legislation that authorized the 6 DCA beyond-perimeter exemption slots required service to be offered by a carrier with a hub at the western end of the route. That's why applications for these slots were originally filed by AA, UA, TW, DL, HP, N7, AS and F9 (all with western hubs or focus cities) but NOT by US. Thus, I don't think the UA/US codeshare matters that much in the decision of whether UA should receive authority for the DCA-LAX route. They have a hub at LAX (the western end of the route) and can already carry connecting traffic from US cities in the east over the UA hubs at IAD and ORD.