I also think window should be first, BUT i can see why the seatbelt is important. When the water rushed in, you'll likely panic and can't get seatbelt off, especially when you have precious cargo in the backseat.
That make sense.
So I'm guessing it's non serviceable item and once it's done, it's done.
https://auto.howstuffworks.com/differential5.htm
Were you really "playful" with all these snow?
If your mechanic can give you that guarantee in writing that X dollars = X more years then yes. But again, age and mileage, a lot of components can fail and probably are already failing so it's not a wise gamble IMO.
I think it's just a matter of specific wording like how they describe the front. ACC will give power. So if ignition is past ACC, then regardless of ignition on or off, you will have power because of ACC.