First point, this trade you are talking about for airfield defence is literally infantry which does less training. Look up the Force.ca Infantry page and compare it to the Airforce Defence Guards basic summary you posted here, and you will see they are basically the same except the Airforce Defence Guards do less. I see no need to create a new trade when you already have all the training for the trade in existence. I also couldn't give a flying fuck what 'element' it is considered, this isn't the 50s and we are all members of the CAF not the Canadian Army, RCAF, or RCN. If it is such a critical role you can easily post a infantry platoon or two to a Airforce base for quick reaction.
Second point, Port Inspection Divers and such, filled capabilities that previously no one filled. The training was unique, and not provided for in a other trade. It does make sense to make a new trade when there is a hole which no one else fills, however in the case of a 'Airforce Defence Guard' we already have a trade that does basically 100% of the stuff and more, we call it Infantry.
Third point, in regards to quick readiness, you can easily task a infantry platoon or two for being quick reaction. Its not like the Airforce is the only part of the military which has ever had to be on short notice. When I was in the Navy my ship, when we were ready duty ship, went from sitting in harbour with only the duty watch on board to sailing at sea in 3 hours (showed up as soon as possible then flashed up and went). It is not rocket science, you order your troops to be ready to move in potentially 'x' hours and that's that.
Fourth point, if this was to become a Reservist trade you realize your quick reaction force concept would literally be shot to shit? Reservists only have to show up once a month, and to expect a force capable of deploying in couple hours short notice isn't happening. People have jobs, other commitments, and you can't order them to show up (unless they call up the Reserves which isn't happening unless something really big happens).