No one did, any more than race car drivers worry about the tire pressure specs and what not in the glove box manual. Guitar amps are not the polite table radios the tube specs are written for.I think maybe in the 60s Gibson weren't so worried about keeping operating conditions strictly within the maximum specified ratings for the tubes.
As you do things to the amp that bring back its gain, then any noise it makes will be amplified more.
You can easily have more than one source of hum going. Get the scope going and determine which is 120Hz and which is 60Hz.
I see no reason to unwire the heater CT, then make up a virtual center tap to replace it. The whole idea of the two resistors was to make something when there was no center tap.
Moving the HV CT to the filter cap can't hurt. But your filter caps don't appear to ground at the same place as it is now.
Your drawing shows input jacks, but I see a AMP connector which looks to take the input jacks elsewhere.