Thanks for all the great help. Leak is finally fixed and my experience might help others.
I ground out enough of the tile floor underlay just above the pipe to get room to get a vice grip around the fitting. In trying to cut out the broken off male thread inside the T, I misjudged badly and cut through the inside of the T thread in two spots…what I thought I was seeing as the male thread was really two threads joining. Dumb thing not to have practiced on a scrap first
I got Hercules Blue; their tech support at their web site said to let it set up for 20 mins and then overnight & it might hold the cut threads. It didn’t. I didn’t want to take off the T as it had another run coming in which I would have had to grind off first and then rerout assuming I could get the T unscrewed without opening up a leak further down the line
As a last resort, I wrapped many thicknesses of Teflon on the male and upon screwing it in found that it would hold the water in spite of the cuts
Another thing I found in trying to follow up on Sylvan’s suggestion for a TP fitting for a cut 3/4″ brass pipe was that nobody carried it; it has to be brazed, not soldered which I assume is the reason why. However, they showed me a metal Dresser Coupling which in effect is a compression fitting for 3/4″ threaded brass pipe which doesn’t need the threads, so if I ever have to cut into a pipe and can’t for whatever reason rethread it, this seems like a good solution. The hardware store also sells a cheaper plastic one, but I’m not sure if it would work on hot water plus it was a lot bulkier