Your US domestic flights should be in A if there's A availability on the flight.

You can change your seat assignments on most OW carriers in business if you call the airline directly (it would probably help if you can get each airline's underlying PNR). It's only BA that refuses to do this for anyone except full fare J.

I assume someone else is paying if you are booking a DONE4 from LHR? You are aware that you could save over a thousand quid by popping over Sweden and starting there?