Success, tentatively!
I let the multiple coats of corona dope on the arc location dry overnight, installed the insulating board in the top of the HV box, and put a rope of high-voltage putty around the flyback coil:
In the picture of the insulator, wires are visible under each end of the board. It is a copper-clad board on one side, so I soldered those wires on to ground the board under nearby screws; they will also hold the board in case the adhesive holding it fails. In the flyback picture, the arc location is at the "3 o'clock" position, to the right in the picture. That was all the putty I had, or I would have used more. I plan to replace it with silicone permanently in any case.
Before I applied any power, I did a run-through of the whole process of possible actions and adjustments, and I found that my horizontal linearity coil (also called horizontal tuning, and horizontal efficiency in later sets) was frozen. I had to remove it and apply heat from a heat gun, but it freed up nicely. (Thanks to Tom, Electronic M, for that tip from his own CTC-4 coil, in another discussion I found.)
With the coil reinstalled, I put my Simpson 260 inline with the H output tube's cathode, put my HV meter on the anode wire to the test jig, connected the Variac, and increased the voltage in moderate steps. At 80-90 volts, the set slowly came to life, and I watched the HV go to 20kV without any problems. The H output current was low, perhaps 120mA on the meter even with the Variac at perhaps 95-100V, and I did a brief adjustment of the H linearity coil without seeing a big change. At this point, the set had been on for 4-5 minutes, so I shut it down to check the flyback temperature. It was totally cool on the perimeter of the HV coil, including at the arc location, with a bit of warmth in the "spoke" area of the "tire".
I think I will replace the Simpson 260 with a new meter (still keeping the 0.1uF capacitor across the test leads) and try another test, also maybe running the AC voltage a bit higher, before saying the test was good and preparing to coat the flyback coil with silicone.
One question I have: Should the HV adjustment control give me a linear range of voltage, even with the AC input low, or does the HV regulator circuit function more as a limiter above a certain voltage? Are there any dangers to setting it too low while testing its function, if in fact I can? The HV got to 22-23kV already even with the AC input at perhaps 100V or less. I appreciate any advice here.