Roots have delicate tissues as well as microscopic hairs and symbiotic fungi to take up water. Peroxide will damage all these.
Your plant is probably still salvageable. Ray's advice concerning the heat mat and kelp supplement may speed up recovery. With the peroxide treatment to already unhealthy roots and the change in substrate, expect all the current roots to slowly die but hopefully not before they're replaced by new roots. Sphagnum doesn't work well for me so I repot to bark, if the plant's roots are healthy they weather the change fine, if they're already in poor shape I get a lot of root death.
I'm not sure how you currently have it potted. I've tried the Better Grow media and it looked decent, but my Phal didn't do well in it (I didn't really know what I was doing back then so it may have been more my fault). You're going to want to have your Phal in the smallest pot that can accommodate the roots and it should have lots of ventilation. I get the slotted clear orchid pots from
rePotme Orchid Supplies (you could fabricate something similar with a plastic cup and a soldering iron). With new bark, I've found soaking works better than letting the water run through. I put the slotted pot in a larger cache pot, water until the slotted pot is almost submerged, let sit for about 5 minutes, and then let all of the water drain out. Under my growing conditions, I end up watering every 5 to 7 days (I go mainly off of root color and weight of the pot, but the skewer method was really helpful when I started out).
If you have a larger pot but very few roots, you may want to consider packing the bottom with something like styrofoam packing peanuts. Because the roots are not in good shape, they're not taking up a lot of water from the bottom of the pot so it's staying wet. You want to shoot for a situation where your substrate is near dry by around 7 days.
If you end up repotting again, I'd mess with the roots as little as possible. Even if they're dead or dying they still may get some water to the plant through wicking. I haven't tried the heat mat or kelp max route, but the rootless Phals I've rehabbed have taken a year or two to recover depending on condition at the time of intervention.
Everyone's situation is unique. There are many possible watering methods, substrates, pots etc. You may have to experiment a bit to find the best strategy for you.