There's a good reason just trying to add solder to an old, wetted joint doesn't work - contamination. The only way to make a good solder joint is to start with clean, properly fluxed pipe and fittings. So, if you didn't take that fitting off, clean everything up, flux then try to solder it, that is why it did not stop leaking.
Having a radiator from a different zone would certainly affect the thermostat and ultimate comfort of that zone. Whether you'd get enough heat in there with what you have if you eliminated that one is hard to tell without knowing the actual heat load. As to whether that's a viable way to remove it, also can't say without seeing it (at least I can't, maybe a pro is following your better than I).