You should post pics of the scale rot you speak of. It doesn't really sound like scale rot to me as it seems to be on the snakes back and not mainly the belly, is this the case?
As to covering the lid that can be some trial and error but I don't see how you created such high humidity so quickly to cause the new issues. Were you having an issue with the glass walls of the cage building humidity and creating heavy moisture build up?
Did you get a IR temp gun to check the temps of the cage? In particularly the ledge you created as a hotspot?
The "hospital tank" may or may not be a good idea. I fear that you are going from one extreme to the other instead of making the small adjustments needed to get the husbandry correct.
You need to spend the money and do things right. If you are unable to provide the correct humidity with misting yourself you need to get a MistKing system of Fogger with a ReptiTherm unit to monitor and turn on and off the systems as needed. You also should place the light on a proportional tstat so the light is providing just what is needed at all times. I think doing those two things will put you in a good spot but I certainly will not persuade you to not also buy a better pvc cage.
Trying to keep more advanced snakes while using techniques and equipment not suited for it is going to be a up hill battle. Its always cheaper to do it right the first time. Spend the money now and get it done right. If you don't you will end up spending more on vet bills and eventually the right gear.
Im sorry your having this much trouble and we need to get things right so both you and the snake are in a better place.