I would not put hydrogen peroxide on the snake unless the wounds are fresh. If they are scabbed over, I'd skip the peroxide now.
About all you can do for this snake is to keep it warm and feed it twice a week. For temperature, put it on a day/night temperature cycle with the cool end of the cage around 80 F and the hot end 85-86 F during the day. Room temperature is fine at night. Food can be fish, frogs, tadpoles, and earthworms.
Within 6 weeks, your snake will shed. The scabs on the wounds will be partly or completely torn off. That is when you can put some dilute peroxide on them. Further sheds will occur roughly once a month. At the first shed, the wounds will look pretty gruesome, but you will see improvement with every shed.
I'm not too worried about the wounds. Snakes are incredibly resistant to that sort of thing. What worries me is the inability to move the tail. A broken back is likely to be fatal. Sometimes a vet can treat that sort of thing, and sometimes eaven a vet can't halp. Keep handling to a minimum to avoid more damage to the back.
Oh, yes. Don't use any more mite spray. The snake mite is not native to the USA, at least not outside pet stores, so he doesn't need any mite treatment.
Otherwise, check the online care sheets for normal care of a garter.
Good luck
Paul Hollander