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On your second point, that was my the same issue I had in my discourse with durasteel, You can't have mages completely independent and doing their own thing without any form of oversight. As I said then; I would rather inconvenince mages a slight bit by requiring them to come to the tower for training/checking in/social activities a couple of times a year, than having to pass through villages that were already destroyed by demons or abominations because we didn't want to ask the mages to keep in touch. No my system wouldn't entirely prevent such occurences, but I think it would greatly reduce them.
The destruction of villages seems to fall into three categories. The first is the emerging talent issue, where a child blows stuff up out of ignorance. The second is the desperate fugitive issue, where the templars back a mage into a corner. The third is the magical transgressor, who through incompetence or evil brings harm upon innocents.
The first category is one which is, as far as I can tell, somewhat inevitable. There will always be farmhouses lit on fire by accident, both by magic and mundane reasons. I think that the friendlier you make the Circle system, providing guidance and structure instead of imprisonment, condemnation, and edicts, the more likely the kid and the family will be to come to the Circle at the first sign of magic, so some of the more disastrous outcomes may be avoided thereby.
The second issue is altogether avoided by making the Circle optional. No more fugitives.
The third issue seems to be the one you're most focussed on, despite the fact that it seems to be very rare, at least based on what we've seen. I also think that such incidents are bound to happen, even if they happen rarely, and since they cannot be entirely prevented it seems that the best way to deal with them would be to have mages and templars working together to respond to such an occurrence as quickly and as effectively as possible.