..... The image that you are getting right now: how bright is it and how sharp is it and how much greyscale definition is it???
I'm not that comfortable with the thought that the problem is principally related to low HV...
Now, if you have swapped out the CRT and have the same result, you know the issue is in the chassis vertical section. Two thoughts if you haven't done so already: try swapping out the tubes in the vertical section. If no change I'd Think outside the box and try changing capacities of caps in the vertical sweep. And I'd begin with mica's..... Yes I know you've checked stuff 30+ times visually, but there is something fairly basic going on and you should have confidence in and start trusting your gut. Ask me how many times I've worked my ass out of goofy, arcane weird problems in both radio and television restoration work like this. Something not visually obvious is off. Most likely a capacitor. Check each cap you replaced. I've run into plenty of perfectly good looking new caps that were bad. And as for micas--- I have little faith in originals anymore. Just sayin'... And of course take a break and have a nice drink (a good Bombay martini is my fav and never failed to appropriately clear my brain for serious thinking) and the problem will just whack you right in the face. You're very close. This is not a huge issue. Just think you're looking under the wrong slimy rock (like all of us have on occasion...)
For a "fer instance" last fall I bought a Predicta from a guy who spent forever trying to figure out a (vertical) double image issue. I put it on the bench and after verifying everything was "visually " correct determined the vertical oscillator was just running too fast. I really didn't give a rats ass why -- everything "checked out" so I just changed the resistance to the hold circuit and the problem corrected itself. After over 5 months it's running like a Swiss watch. Just go with your gut and use your experience!!! It's something easy...
George