It is fairly convincing that R429 is broken and responsible for the vertical deflection errors, you can check by measuring for resistance from the ends of R427 and R428 to ground. It should be a single turn trimpot so the fact that it continues to rotate is an indication that the adjuster is not moving the wiper and possibly the wiper is completely detached leaving no connection to ground.
Either way you need to find a replacement for this 250R preset that has about the right spacing of its fixing pins. Did you make a note of the position of R455 before you tried adjusting it? If so then put it back to that position as a starting point for your adjustments after replacing R429.