Now I'm confused.
I've always heard that higher humidity is better to treat a RI. Not wet substrate, just higher humidity overall.
Mites won't kill the snake, neither will eye caps.
Treating the mouth rot and RI is absolute first order.
You "can" treat for the mites as well, just make sure you don't overwhelm the snake's system. Any mite treatment IS poison. Its just "save" if used correctly.
But you can do damage by doubling up or using to much, just like what can happen with dogs and cats if you use more then one flea treatment. So pick something and stick with it...if you want to try something else, let a few days go by or ask the Vet and on our forums whether that combo is safe.
Last thing you need is to add neurological damage to the snake at this point ;)
Don't get overwhelmed. The snake didn't get into this bad shape within a couple of days (no matter what former owner says) and it will take more then a couple of days to fix it back up.
But you will get there. Slow and steady !!