OK, so a couple readings and waveforms with the analog scope today.
Double checking pin 7, V4...
With video: 0v
Without: -3.6v
Scope:
B4 V det:
https://youtu.be/7qv-o0B_aHU
After V det:
https://youtu.be/GdDLbR7zWb0
Pin 7, V4
https://youtu.be/XtraunfLyd4
Pin 2, V4
https://youtu.be/Yiq7CfYEB8s
Replacement Nuvistor ordered, as the current one has bad grid leakage. Just a hunch, there may be excessive noise coming in from the tuner AGC because of this, combined with the boost voltages applied later in the IF, could be distorting the video waveform at some point.