aefron88

Impact gun would be my first thought. The nut isn't staked is it?
Otherwise you could pull the cvt cover off and try getting a big strap wrench to hold the secondary from turning.
Assuming your 1 way bearing is working you could also remove the spark plug, put a rope in the cylinder and jam it close to TDC. Put it in gear and that should keep the driveline from turning.

If you have a powersports shop that works on clutches you could get them to put heavier rollers that will keep the cvt in lower "gearing" at the expense of top speed.

Normally yes it touches coolant and you'll need to drain below that level and refill after replacement.
When you refill the radiator, these things can be a bear to bleed all the air from the cooling system. Any air left will lead to more overheating issues. There's a plug on the right side of the engine where the rad hose goes in that you can partially loosen while running to help bleed air.

That means the tank is empty.
First make sure there's fuel in the tank.
Next lift up the seat and check the wire connector going to the fuel tank on the passengers side. Make sure its not loose.

I've never seen a spec published anywhere, maybe someone will chime in.
I doubt theres enough overhead to run an electric heater that puts out enough to be worthwhile. My experience with these types of things is there might be 10-15 extra amps of overhead MAX at idle, quite possibly less. A 100 watt light bar will eat half of that. A 100 watt electric heater would eat the other half and isn't going to heat the cab.
If you feel like experimenting you could keep adding load until the battery voltage is <14V at idle, that will give a good idea of the max limit. Dont forget to turn on the factory lights (35W ea), brake lights (10W ea), turn signals (10W ea), etc as all that has signifigant draw.