Just a quick 'opener' Tall-boy'. Do you have your uncle's Record of Service and is that what you are quoting from regarding dates etc?
If you don't have the service record, it will answer most or all of your questions and will prove to be a valuable document for all other research you want to do about your uncle.
Don't get too hung up on mid upper or rear gunners, as the two often sorted themselves out when going to an operational sqn. The flight engineer - who probably had very little actual flying experience during his training - joined at HCU as did (usually) a second gunner.
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