Trying pre-killed before live is fine, it's not like you have to get her eating now. My Pinstripe I brought home earlier this week hasn't eaten in ten months and I'm not worried to take my time getting him back on feed since he's not deathly skinny like my other two were when I "rescued" them. Are you positive there aren't any other rodent suppliers closer to your home? Sure would make your life easier, lol. Fresh killed is your best bet as a next step, then if that doesn't work try live. If you can get her eating live regulary you may have success switching her back to pre-killed or f/t eventually. The main concern is getting her to eat regularly, kick-start her feeding response and get her on a schedule.
You're right though, the analog hygrometers are crap. They don't read near the actual percentage. My first normal came in a small tank with an analog gauge that read about 30% off of the actualy percentage.
As for the substrate, if you're worried about impaction, switch to newspapers or paper towels and feed them in their enclosures. I prefer using paper towels because you can see any possible mites crawling on them and they're easy to clean, just dump them an put down new ones. You might actually get a better feeding response from her if you feed her in her enclosure where she is most comfortable. If you really want to keep whatever substrate you're using, I suppose you can take out the water bowl and lay down paper towels during feeding, but I'd imagine that'd be too much of a hassle in my opinion. I prefer being able to shut the tub and leave them alone after they strike with no worries of impaction, then I replace their water bowls once they've finished.