Your cool side temps sound fine. And yes, when snakes that have mites soak, they often leave a few in the water bowl.
I've never personally used Reptile Relief but I've heard good things & would try that. Be very careful to follow instructions if you try "PAM" (Provent-A-Mite)- that's proven toxic to some, especially if instructions aren't closely followed. I've never used that either. I prefer methods that are least toxic, which is why I first would do a daily soak (w/ drop of mild dish soap) if any mites are found. That at least will get rid of MOST mites, & won't harm your snake one bit. The Reptile Relief might (with luck) be all you need to finish the job, along with careful cage cleaning, after that. I hope so, anyway. Current treatments- see the link in my post above from another long-time very experienced member & fellow mod here. I'm happy to say I've only ever had 2 snakes with mites, & that was long ago.