Strangely, CRT pin 3 is giving 1480 V in the disconnected G1 cable, while pin on the H.V psu circuit is also giving out about the same HV voltage.
Combined your symptoms suggest some kind of improper connection between hi-voltage generator and one or two horizontal plates (or else the horizontal sweep generator.) Such a thing is unlikely and more troubleshooting is needed.
As a result the hi-voltage gets lost in the horizontal drive, or vice versa. When you separate them, one of them starts to work but not the other.
By the way, the electron beam should be
negative polarity relative to chassis, or ground. (My scope hi-voltage is -1000 VDC, yours should be -1480 or -1750).
Things to look for:
* improper wire connections
* high voltage wire touching components
* build-up of carbonized dust and lint improperly carrying hi V to horizontal drive
* blobs of solder on circuit board, bridging across copper traces
* misplaced components
* component leads bent so they touch other components
My own scope gives me recurrent problems with arcing of the high-V circuit. It occurred first inside the main power transformer, then in each extra transformer which I installed to create 6.3V heater voltage for the tubes. I want to make a substitute hi-V source (using Cockcroft-Walton voltage multiplier), but I'm not certain it would succeed.
If you have sufficient knowledge, you can try making a hi-voltage source for your electron beam, and disconnect the internal one.