The Brits might have put Merlins in their P-38’s — it was the Brits after all that put the RR Merlin in the P51, and that made that fighter a world-beater.
I’m not aware that they did, however...
Actually that would no doubt have been great, but unfortunately the only P-38s that went to the RAF were a few which were delivered without the turbochargers, and they were dogs. They were known as "castrated P-38s" and left such a bad taste that they didn't ever ask for any more. The USAAC didn't have a lot of success with the P-38s in the 8th AF in Britain, even though it did very well in the Med and Pacific theaters. Apparently the much colder conditions in northern Europe caused a lot of problems.
Don't think so, the supercharged Allisons worked fine and there were other places for the Merlins to go.
P-40 "F" and "L" ran Merlins without much improvement in performance...Google tells me that those were later re-engined back to the Allison.