[MrFixr55]

When you get back to it (and I definitely understand why one must walk away), the first thing to discern is if the motorboating is just as loud with the volume control at minimum as it is at maximum? That is a "Divide and Conquer" piece of information.

The "actual" C101C does appear connected the same way circuit-wise that the "dotted line" C101C appears. I don't know if adding the 150KOhm resistor between Common and Chassis will help, but it couldn't hurt. Adding a 0.05uF cap between B+ and common may be helpful.

C302, the bypass cap for the screens of the converter and IF seems to be too low a value. In earlier model Transitones, and most other AA5 radios, this cap is 0.05uF.

Did you try the radio with the 12AV6 jerry-rig in place? If not, try it. If it works, maybe the 14B6 is bad or there may be a bad connection between the cathode of the 14B6 and Common. Verify the continuity between the pin and Common. If the plate resistor is actually close to 470KOhm but the voltage is that high, then the tube is not conducting correctly.