Ideally, you'd want to heat the drop elbow enough to expand the metal but not enough to melt the solder. You could use a fresh piece of solder on the existing sweat joint to evaluate when you're getting close. When you DO get close, a cold, wet cloth on the valve body will quickly cool it relative to the elbow.
I'd be surprised if that valve can't be disassembled somehow. They had to assemble it somehow when it was manufactured.
I'd be surprised if that valve can't be disassembled somehow. They had to assemble it somehow when it was manufactured.