SSB, I agree with much of what you say, but I think breaking into the East Coast-London market will br incredibly difficult. Whatever they choose, B6 will find LON-NYC and BOS incredibly competitive. Between the frequency offer by BA/AA (up to 16 flights each way, each day), 9 flights each way with VS/DL and low fares offered by Norwegian (twice daily) to NYC alone, one wonders what the gap in the market really is? Would STN, jetBlue and Mint alone be point of difference enough for success? I agree that Primera proved that even the 321neo can reliably operate to STN from the East Coast, the LR or XLR would, of course, be better, but I think that's a bigger risk in announcing now. I just can't quite understand why they are giving their competitors 2 years notice of their plans. If they wanted to, AA could configure some of their A321neo for Transatlantic service for Summer 2020, or even the winter 2019 schedule and have JFK and BOS-STN up and running before Airbus even begin to assemble jetBlues first 321LR. AA could easily use these aircraft on South America or transcontinental services without configuring them back, so its pretty low risk.
With such a delay its probably best to keep it to "London" rather than specify an airport.