Odds of a refueling stop in May are incredibly low. The jet stream is generally much more calm around that time so the headwinds aren't an issue.

The CO planes have AVOD and power at every seat. And if you pay a few $$ extra (or have CO/UA status) you can get the E+ seats, which are ~38" pitch.

SAS has personal IFE as well but no power. If you can get in on the OptionTown (I think that's what it is called) thing to get upgrades that might be a consideration as well.

I'd pay the extra DKK150 to avoid Heathrow without question.

Personally the single/twin aisle thing just doesn't bother me. The seat is basically the same and I'm not going to be sitting in it, not running laps, so I just take the plane that has the better schedule/price.