if you put a resistor over the cap, you wont have DC restoration anymore
the resistive path to common is now through the 2 series 470k resistors.
unfortunately I do not have a cathode-video set to experiment with, however, i suspect the tearing problems are due to the circuit interfering with the synch separation, which also comes off the video amp in your set.
anyhow, thats the basic idea:
a diode to clamp the black level
capacitively isolate the CRT grid from the DC level of the video amp
set the DC cathode level with a resistor divider
the details are the complicated part!
go even to 10k at r5. without a similar set, I cant do much other than consult :/ do you by chance have a scope to post some waveforms? as a last resort, i can try to simulate your entire video amp circuit, but i wouldnt have a lot of faith in the tube models up in the Mhz range, if the models even exist.
for what its worth, the 48 RCA 6t45 chassis has dc restoration, and even it doesnt work thr greatest. I took some captures of thr cathode waveforms with all black and all white screens -- i will post them over in the 6t75 thread. But the front porch "black" level on the CRT grid drifts by about 10 volts from "all black" to "all white", causing all black to look gray. That set uses a positively biased triode grid to get diode action from grid current - and it probably doesnt make for the "tightest" clamping.
if anybody else has ideas, id love to discuss. seems like an interesting little project.