I used to be chief engineer at a Sony service dealer, I've repaired thousands of Sony CRT sets.
I've no idea what you mean by a 'damper diode'?.
That's EXTREMELY bizarre - they don't usually go S/C, and it wouldn't make the set trip anyway - I'm bemused you even tested it.
Never an issue in Sony sets, as they use higher quality components than the cheaper brands - I've never known one fail in a Sony.
LOPTX's DO fail in Sony sets - they also commonly arc internally and take out the LOPT transistor with them - if you change the transistor it blows instantly again. Depending on the chassis, and particularly if it's 16:9 or 4:3 (even if the board is the same) it's very common for them to take out the main micro-controller, which is a large SM device.
For a start remove the LOPT transistor (so the line can't run), and fit a 60W bulb across the 115V - this should light the bulb dimly, and the PSU should run with no issues.
I presume you have the schematic?, with the LOPT transistor in place check the various trip circuits to see which one is making it trip.