[Electronic M]

If the short (G1-K?) persists without heater voltage applied it might be worth while to take something like a 20+ Amp low voltage lab power supply with current regulation, connect it between the two shorted elements with the heater completely cold and see if you can burn the short open with high current (a supply with current regulation would allow you to creep up on the fusible portion of the short and in a dark room give you warning via red glow if something else was about to burn open). If the heater is cold then theoretically there'll be no cathode coating emissions and no cathode damage and all the current will be through the short....Granted it's gambling on the short having less material than the rest of the internal gun structure and if that 50-50 gamble loses and you don't see glow in the wrong place in time to stop you either burn open an internal connection to the cathode or an internal connection to the grid.

If the tube is completely unwatchable it's a gamble I'd be willing to take assuming you've tried all gentler methods first.