Before accusing the HV transformer ... some remarks.
How "very low" was HV at the test point? Is the oscillator amplitude at Q1430 also (in ratio) very low?
What voltage did you have at Q1423-emitter (or pin "P")? The shown voltage -4.35 V is not more than an indication; also there is ripple of 1 V or so. The oscillator starts with base current to Q1430 provide by R1425. Amplitude increases rapidly when Q1423 conducts at the negative going peaks of "P" voltage (at about -1 V). With too high load that situation might not occur.
Did you check HV pot R1401?
Did you measure the current via F1437? Normally about 0.27 A.
It might be possible to test the transformer (and load) to some extend. Remove F1437. Apply + 10 V (w.r.t. GND) or so at C1437. Apply about -2 V at "P". Apply a signal generator output (via DC blocking cap) to Q1430 collector, keeping the amplitude below about 4 V. This way no semiconductors will start to conduct. There should be resonance at about the normal oscillator frequency, about 25 kHz. Turns ratios can be checked when when you can access the secondary winding ends or rectified voltages. (Just an idea, I have never tried this in my 454.)