One tub fits about 3 medium rats (floating on top in one layer). So you can do more than one rodent for sure!
I think the biggest issue with your current defrosting method is the potential to cook the rodent, not the lack of heat.
Also, if they refuse once, I wouldn’t offer again. Leaving it for a while is ok. However if you keep offering and they are not interested you are just adding stress for them.
Seriously. Calm down, deep breaths, and leave everyone alone until next feeding day.
Think like a snake. Your stress is spilling over to them. Give yourself and them a break.