Heat transfer to adjacent solder joints behind wall

Use a piece of scrap metal (old beans can etc) to keep the firey blasts of your torch from hitting the other pipe.

If the scrap metal has paint or varnish pre-bake it out of doors, to avoid releasing noxious fumes indoors.

Fiberglass insulation will also work.

I don't see any reason why unintentional reflow of the solder on that nearby elbow would inherently be any problem. Try to minimize disturbing it while the solder is molten -- don't pull the pipe out of the elbow, and hold it with pliers so the pipe won't rotate while you're trying to work the other fitting off the same stub of pipe.

It'll probably work out just fine, and if you have the parts and tools on hand you'll be ready to deal with any trouble that might arise.

The biggest thing is to make sure you keep all the water cleared out while trying to solder. Have a dissolvable sponge (such as a slice of white bread) on hand to stuff up into the pipe to hold water at bay, just in case.

Any particular reason not to just cut the valve out and add a new one with a few repair sleeves or sharkbite connectors? Problem with heating to remove the valve, you still have to spend time cleaning/emering off the old solder from the pipe so it will fit into the new pipe fittings. I have found it takes less heat to solder a new joint than to un solder and clean an old one.