To answer question 1 based on my experiences...

United may have award seats available for its own customers. However, United may limit the award seats available to partner airlines. I had this problem trying to get my wife a business class award seat to Bangkok about a year ago. She had a C seat on all segments except one (SEA-IAD), where she could only get Y.

We had booked about 8 months out. Despite weekly calls to US and occasional requests by me to UA to request a seat ("inventory" or something was the term...it was suggested to me by other forum members), we never did get a C seat for that segment. In fact, I saw UA 1Ks, 1Ps and 2Ps getting upgrade seats at the gate, so it seems even walk-ups from the UA program outrank award requests using US miles.

In short, in my experience it's up to UA to release the capacity to US so you're pretty much at UA's mercy, code sharing or comarketing or co-bankruptcies be damned.