Well if AS/QX metal don't fly to the international gateway in question then it's obviously impossible, but otherwise, they will put you on AS.
Here is an update.
1. Called Alaska this morning. They said they could get me from LHR to MIA on July 6 (a day or two earlier than I wished for). I said "from Miami to Seattle I would then be on my own" and the rep said "correct, there is no space." I did not press for details "is this because there's no space on Miami to LAX on American, or what about going the next day on your direct flight from Miami to Seattle that leaves at 8:30 am. Or how about we find our way to Orlando and then we get on a flight, could you do that?" There's a point at which asking questions seems futile, especially when 3 other reps on 3 separate days have said the same thing: we can get you across the Atlantic, but we have no flights, even on other days than which you land, all the way back to your starting city.
2. The rep said "time to start looking at a complete trip using other airline partners." She searched extensively using DL/KLM/AirFrance and could not come up with anything. The entire trip, not just the return, would need to be booked on either all Delta, or on KL/AF. We can not mix and match, using American Air one way and then DL/KLM/AF the other. I am not sure if we can use DL one way and a flight from either AF or KLM the other, my sense is no but it is not an option anyways. We did not pursue British on this trip because the fuel surcharge is quite high.
3. In general, my advice would be:
A. Do not count on a return flight being available even if you have the outbound. Have a backup plan: it could be the willingness and budget to pay for the return leg and realize you're paying both the cash price and also full miles for a half trip, or have arrangements on another carrier's miles program for both ways and then you can redeposit your miles back into Alaska. Or find a carrier that offers one way tickets and mix and match, for example, one way on American and its partners, and then the other way on United and its partners. Unfortunately Delta appears to not have one way tickets and also to have the worst award availability, even at medium award ticket levels.
I hope this advice is helpful, and I hope some day I'll be able to post a follow up story with a happy ending.