cyfler, the last member who posted his story IAW the OP title, Danny42C, has sadly passed away and with him a direct link to those dangerous and challenging years. Nonetheless the thread continues to honour those who told their story and to add to our collective understanding of the colossal organisation that the Royal Air Force was then. So your post is both welcome and apposite and I thank you for bringing this unique thread back onto page 1 where it belongs.
Danny could have rattled off answers to most of your questions I suspect, but I have to fall back on Google to seek answers:-
ASU translates as Armament Support Unit in this much later post, but could he have started out in such?
History of the RAF Police at the ASU
No idea about T Sergt (Temporary, Technical?) whatever it was he goes back to AC2. I suspect this might have been disciplinary move (ie busted to the ranks?), but again not sure.
He claws his way back up, via LAC to T Sgt again, before starting pilot training.
311 was indeed a Ferry Training Unit:-
311 Ferry Training Unit[RAFCommands Archive]
I suspect the IPBD is 1 BPD, ie No1 Base Personnel Depot, as here:-
https://forum.keypublishing.com/foru...tions-question
No idea about the supy's, possibly supernumerary due a missing unit (ie its moved location since the posting/attachment order).
Well, there's a starter for ten which will no doubt be heavily corrected by those more knowledgeable than I. Good luck with your research! I suggest placing RAF after the various acronyms, as it can help sort out the search better.
PS I think perhaps that missing means just that. He is missing 13.7.44 which is the date he is later presumed dead. The intervening supy's are probably an administrative procedure whereby he is kept on strength until finally presumed dead.