[Yamamaya42]

Again, given the limited tools you have on hand, it's very unlikely that a complicated issue like this can be worked out.

It's like trying to play Jenga on a crooked table with your eyes closed.

Vertical sweep is dependent on the boost from flyback transformer, so that has to be working, high voltage is dependent on horizontal oscillation being correct and at the right frequency, amplitude and waveform, and if it's NOT , the horizontal output tubes can go into oversaturation, overload the power supply and rob power from critical systems, like the horizontal oscillator.
Or, something that controls the horizontal oscillator can be in play here, the signal shown on pin 9 of V12 is key to timing and control of the horizontal oscillator, generated by V10, I'm not sure what effect the removal of V6 has on V10, but the signal path goes that route.

But as I and others have been saying, with just DVMs, you will be very unlikely be able to crack this nut.