Thanks Paul!!
Unfortunately, I'm stuck scratching my head now... I had -1kV with the "new" transformer immediately after installation with the multiplier disconnected (anode and multiplier ground lead disconnected), so I re-connected the multiplier to check and see if it was good since they're hard to come by. I was still getting over -900V with it connected, so I chose to go through the troubleshooting procedure once again, assuming that the multiplier may be fine. Now I'm not getting more than -750V with the anode lead and ground disconnected. I'm starting to question whether the transformer was ever the issue, or if there's multiple failure points in the circuit. In addition to the 465B (and 465) troubleshooting charts, I've lifted a lead on every diode and checked them, wholesale swapped all of the transistors (then swapped the originals back in since nothing changed), replaced the HV diode, and of course replaced the HV transformer. At this moment, I'm unsure of what is loading down the circuit.
If I am to believe the -2450V troubleshooting flow charts, then the HV transformer is bad. I'm just not quite sure where to go from here.