I am honestly puzzled - about your carrying capacity. All you can carry on a snowmobile is a hefty day pack (50L capacity or approximately 3,000 cu in))? I have no experience with snowmobiling, so this is an honest question - please enlighten me.

My cold weather snow experience is ski touring and climbing in similar temperatures, but on any trip, we would carry packs of up to twice the capacity you list. Those would be adequate to carry the gear proposed here. Isn't there any way to affix duffel bags or similar to the snowmobile?

One thing about a sleeping bag - consider it part of your FAK, since any significant injury will not just be the injury but the injury plus potential hypothermia. You need to keep the victim warm....