Later on in the war they were very picky about who got full wings on heavysISTR That they began giving the Flight Engineers enough training to get the aircraft back if the pilot was incapacitated.
( the training schools in Canada and the US were in full swing by then)
if you got scrubbed late in the course you could be re rostered as a flight engineer who latterly was expected to be able to take the place of any crew member ( including the pilot if possible) . My grandads flight engineer soloed on his pilots course and then developed a slight prob with one eye, scrubbed but then became an engineer.( and occasionally flew a bit when back over the oggin to keep his eye in so to speak , and so grandad could have a slash).