I'm not sure I quite understand what you are saying about your setup for the winter. You state that you are "driving through a Toronto winter with OEM 18s". Does this mean that you also have the OEM Michelin Pilot Sport 2 tires mounted?
If so, then this explains all of the problems you state you have had with the tail kicking out and the car being generally scary in the snow. As has been stated many times before on this and other forums.....Pilot Sport 2's absolutely SUCK in snow and ice conditions. They are not designed for that environment and the rubber compound just hardens up and becomes like a brick in cold weather so it provides almost literally no grip. If you are indeed still on your Michelin PS2s then consider yourself one of the luckiest people around for having made it through a snow storm on them.
Now if you meant to say that you are running OEM 18s with a winter tire mounted, then I am at a loss as to why you are having so much trouble keeping the back end of the car from kicking out
. I would definitely start with staying away from MDM and driving the car with all driver aids on and as sedately as possible. I have a set of Bridgestone Blizzak winter tires on 18" wheels on my E90 and it has driven beautifully through snow and ice. The rear wheels only spinning out a little bit on the most extreme of ice conditions so far and certainly no back end wiggling without intentional prodding of the throttle pedal for some fun. Solid and very controllable is the best way I can describe it.
I don't know if this helped. Maybe if you clarify your setup for winter driving I can make some more relevant suggestions.