For the earlier flight, why would AA turn the plane around when it was very near Keflavik and fly three hours back to London rather than landing in Iceland for medical care?
Because who wants to be stuck in Iceland when you can be stuck in a hub. (BA and AA are partners and it would be fairly easy to move passengers to the US in the off season from London. Iceland may have required an empty plane sent from the US if the aircraft or crew had to be taken out of service.)
Maybe guarding against the possibility of contagion.