A manic would/could have been caught by the mental evaluation required for gun registration and ownership within the Canadian system. A system you failed to mention and seem like never considered. No doubt as you didn't know about it.
Your scenario is a loaded one. A shooter could take more careful aim thus increase the chances of mortal wound; 1 shot, 1 kill, a marksman.
Changing a mag can be very fast and done with ease with some practice. Sure there will be a delay but that delay is of mere seconds.
America isn't Canada. America isn't the UK. Look at factors such as culture and history not merely gun laws. Canada has less shooting because it does not have the same cultural and historical issues America has.
The murders happened in the US, where it is easier to get a firearm. It was stated that the AR-15 is legal in Canada. My argument stated way back was that the big problem was magazine size, not the legality of the AR-15 itself. This is why I mentioned magazine size. It is not Canadian laws and practices that are under review but US ones. Talking about cultural and historic issues is not going to solve any problems. The guy in question was not a marksman. He came in pointing the easily aimed AR-15 at whoever he saw and kept squeezing the trigger and changing magazines every 30 rounds. He expended 150 rounds. If he had needed to change magazines 29 times instead of 4, fewer people would have died, One might even argue for the availability of 30 round magazines affecting his mental state and contributing to his decision to do this.