Are you seriously telling me no-one knew? Not hitting another aeroplane in good VMC is a matter of lookout both before and during aerobatics. As for never happening again... Regulation alone won't do that, continuing adequate supervision is what is needed, both medical and operational.
Gliding contests are, by there very nature, likely to change. Mostly in the timing. The fact that one was promulgated should have put a note of caution into the AEF operation. My understanding is that there were a large number of competition gliders in the area at the time, so probably avoiding aerobatics might have been a safer option.
Your details about gliders circling three miles from an airfield would put the glider in the open FIR with the same right to be there as any other aircraft. Military airfields have an aerodrome traffic zone (ATZ) like all airfields which should only be entered to land, (or to look at the signals square with a view to landing if the airfield is not PPO). Three miles from the airfield is NOT in the ATZ. A Military Air Traffic Zone (MATZ) has no legal significance to a civil aircraft (at least it didn't in my day), although, as you say, good airmanship would involve avoiding obvious conflict areas.