Ensure soaked lab block works well too. The babies can suck the "milk" out or eat the soft mushy block. Ensure won't spoil like actual milk either.
You could try putting them in with the prego, her hormones might be in effect enough for her to take care of them too, even if she doesn't nurse them, she might hover over them, wash them and such. If not, nothing lost.
The mom could have had birth complications or the peanut butter could have choked her too. Depends on how you offered the peanut butter. Sometimes moms don't make it, especially if she was thin when you got her.
I just picked up strawberry Ensure to use in case I have any rats looking thin or any reason to give meds to them. It works wonders on a rat who looks thin or worn down. The strawberry flavor is supposed to be like crack for rats... but I haven't yet tried it, only vanilla, which they also liked.
I've had no issues at all with my kiln dried pine. Pine bedding like for horses that is NOT kiln dried has a lot of the oils in it and can be an issue. The baking process removes some of the oils, so it's safe.
It's a complete fallacy that someone who is breeding for feeders doesn't care about the rats' health or condition. We want the best quality rats, in good condition so that they grow best, grow big and stay healthy so they don't lose any condition. And a lot of us just want them healthy and happy because we give a poop about the animals in general. Just because I'm growing animals that will be food doesn't mean I neglect them or abuse them. I raised chickens for the freezer and they were every bit as well treated and cared for as my "pet" chickens and breeders.