Late last night I did indeed shut all the water to the house off and drained the line with the stuck valve in it, and I was able to disassemble the valve and get it unstuck. Now it's working again, and we have hot water.
No point. The bronze braze is going to hold, it's solid as a rock.
I'm going to replace this tank pretty quick, and now that I have the shutoff valve working and understand the piping I now realize the tanks are in parallel and not in series so if I get another leak short-term I can just shut this one down and drain it into the floor drain and switch over to the electric hot water heater.
GWE