Heat transfer to adjacent solder joints behind wall
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.