Sounds like everything was covered. I usually pre-treat paper towels with pam and have them already in my quarantine setups just in case.

I would suggest that you go ahead and pre-treat some paper towels in a well ventilated area (I do this on my screened in porch) so that you have a supply of already treated, dry, paper towels to replace your current ones when the snakes soil their cages. The PAM is good for two weeks, so you can treat some, say on the weekend, and have them still be good for the following week.

I would continue the PAM-treated substrate regimen for at least 1 month...maybe 2 even if you see no visible mites.

Good luck! Mites are no fun. I've had an ongoing battle with them due to some (apparent) mite eggs on cork bark.. persistence is your only weapon..If you can keep up your routine for ~1-2 months and be consistent, you should be able to win.