I know this thread is several years old, but I just wanted to post and say that this happened last night to my two babies, and this has been the most relevant post I have been able to find.
I work in a lab and basically the babies were going to be euthanized or I could take them. I took two boys because we have an older boy and I was going to introduce them so they would all have cage mates.
The boy we have now is 13 months old, and I raised him under the same circumstances...got him when he was 11 days old and he made it, just fine.
The two boys we got were 10 days old, because that's when the mom was euthanized. I think they were probably on the small side because she wasn't eating a lot, and so they weren't being fed enough. But we had them well maintained until they died, at 16 days. We had a heating blanket under the cage and made sure it never got too hot, we fed them every 3-4 hours and stimulated them to go to the bathroom (they always peed, usually pooped once a day).
Their fur was coming in beautifully and they had just opened their eyes on day 14 and 15. They were crawling around like normal, then all of a sudden one of them went pretty limp and just started gasping every 5-8 seconds. Like you described, his color was almost turning blue/gray! Then only about an hour later, he passed. About 15 minutes before that rat passed, the other baby acted the same way.
My husband and I were devastated and had no idea what was going on. I had had a female in the past who was choking for a few hours, but I read that they usually work that out for themselves. She did and was fine. I thought maybe the baby was choking and acting a little more limp because he was so small, but I definitely don't think it was that, especially since there was no drooling and the other one died so soon after as well.
I am sorry I don't have any answers to contribute to your questions, I just wanted to let you know as I read your thread the same exact thing happened to me. It was so sad and discouraging. They seemed to be so healthy and happy up until their last hour (literally). I eventually would like to rescue some more rats, but I am afraid something similar will happen
If you happen to read this, I just was wondering if all the other babies ended up surviving (the ones from the other litter). I just ask because luckily we didn't introduce our older male to them (we wanted them to get a little bit more independent/have their eyes open before we tried in case he was a little aggressive with them)...I wouldn't want him to potentially catch whatever they had.