Herpstat should be fine after an outage. They should go right to the setting you have. I have a Herpstat 2 and a Herpstat 4. Both have gone through power outages in my house numerous times with no issue.
As for the frozen rodents, as long as you don't keep opening the freezer, they should stay frozen for quite awhile. The freezers are heavily insulated for efficiency. With that said, when the power goes back on, if it was for any considerable time, I would definitely check the rodents and make sure they have thawed out at all.