A moist feeder is fine BUT...this allows the substrate to stick to it more so your snake ingest more substrate than it should. While the substrate MOST LIKELY won't harm your snake to much can cause impaction.

As for your temps. I highly recommend getting a IR temp gun. If your cage floor at the probe is at 90 the thickness of the substrate will make the temp on top of the substrate very and it will be lower.

If you thaw it in water you really have no need to pre-thaw in the fridge. She is explaining like many will thaw meat like a steak or chicken for cooking. You do it so the meat is ready for cooking and doesn't require you to thaw it all the way when you want to cook it. Saves time. Feeders are so small though the water does a great job thawing them out.