car seats are designed to be safe in a sudden impact situation. Flight safety is more about the dangers from turbulence. That's why standard seat belts are rudimentary and the impact protection is via bracing. So you don't really get more safe using a car seat. Our experience was that it didn't allow the boy to sleep comfortably either, and getting it in and out was awkward. Also the recline from the seat in front caused space problems. So although good in theory it failed the practicality test. We checked it in this time.

Planepal was much better, though theoretically you have to deflate it during turbulence. He slept 8 hours of the twelve in two spells, curled up comfortably. We had written (tweet) permission from BA to show the crew if they were unfamiliar with it, but wasn't an issue.

i don't see why it wouldn't work in WT+ but it's a nice snug fit in WT. It's below the recline point of the seat in front for both.