What are the “must do” tasks before winter hits

Ice/water can damage anything and everything.

A vinyl deck should be OK. Your pavers might heave a bit if water gets under them and freezes. But that's the great thing about pavers. Easy to fix come spring. (tamp it down or add more sand if it needs to be raised)

Given how low you're keeping the apartment temperature, and given that the ambient temperature is relatively warm, I think you can probably get away with leaving it in place. You'll lose some heat, but nowhere near as much as you would if the differential between inside and outside was greater.

Digressions:

I've seen some weird-and-sometimes-wonderful approaches to AC mounting. I've generally used smaller units that could get by without external brackets, but I've seen one (in NYC) which used a steel square-bar to bridge the opening and carry the weight rather than letting it rest against the upper sash.

The one time I had an AC which really was too heavy to lift, it did have outside brackets built into its case (rather than permanently installed to the wall). The proper solution was actually to mount the case by itself and then slide the guts into it, rather than trying to manhandle the whole thing at once. When dismounted, I kept the mechanism on a rolling cart that was almost at the right height, so I could slide it from cart to case and back without ever having to lift the full weight.