Keep in mind that all the filaments are in series. Lowering the ballast will increase the current through all the tubes and the voltage will be split between them in the same ratio as before. There will be very little increase in the CRT voltage.
Video signal makes no difference. I've found that if a CRT is going to wake up, it will do so even at it's normal operating voltage, it just takes a while longer than using elevated voltages.
Rejuvination works by blasting a layer off the cathode. There's not much material there to begin with in these short necked CRTs and it's liable to make things worse.
I hate to say it, but I've encountered about a dozen of these 21EAP4 CRTs and all were deep into the bad. No waking up or rejuvinating helped
