Ive done a few fine winter trips into APP; an amazing experience free of bugs and crowds. Once, in late February, the temp dipped to -33C ...ouch. I had a -25C rated bag and it was just barely warm enough along with some heavy weight underwear and a double ration of whisky.
But you and your -12C bag sh be ok: the average temps for APP in March is about -5.
Also, as all the winter camping veterans will tell you, its as much about what is under you - the insulated pad - as what is around you, i.e., the sleeping bag. I use an Exped Downmat because it is warm and because I am aging. Its a sweet luxury that makes my old Thermarest feel like one of those blue closed cell pad. On another trip, before I bought the Exped, my old Thermarest sprung a leak and I froze my arse off, even with my warm bag.
Re choice of boots - it depends on your activities. If you are snowshoeing and moving around lots, a winter hiking boot is adequate. Gaiters in the wet snow are a great investment too.
You will get lots more info from the winter camping gang on this forum as soon as 5 cm of snow touches down in the park.
Now you got me dreaming about winter camping instead of work. Nice problem to have though.