As others have pointed out, you can use AS/QX to get to the gateway city, but cannot combine various partners in one award. The agents tend to get pretty creative on finding those routes and gateway cities...and don't forget stopovers are allowed on most award reservations...there's nothing stoping you from making a non traditional city (AUS, SAN, MCO) your gateway city and visiting friends on your way... Lastly, there is speculation that we may see one way award options with some partners in the future. I would guess maybe Delta and American, but likely not the international carriers. This might make it easier to find space via one way tickets to Europe or Latin America and essentially do a combined itinerary as separate one-ways...however, this remains to be seen.

As for your second question, I think that this used to be possible, but was removed as an option 2-3 years ago. I'm sure our moderator jackal will pipe in here, as I believe he did the BA award to AUS via London in the past. (Edit: looks like he beat me to the punch).