Onward! Hey.
Give a holler when you've got the 'real' test gear in hand. I was gonna suggest the 'quick & dirty' method of signal injection using the 6V heater supply like you did earlier for the vertical. But with a proper signal genny you can attenuate the level as you work back stage by stage.
Since you've verified the plate voltages on the 12AU7, have you checked the three peaking coils (L10, L11, L12) for continuity? And how about the detector diode? What's its forward/reverse readings on the ohmeter? Not shorted is it?
oc