That really does suck and I'm sorry to hear about the loss. It really sounds like you did your homework and things sounds relatively normal. I've had a couple pass away over the years without any explanation. Necropsy may give you an idea but unfortunately won't bring the little one back. I would advise to do a thorough cleaning of the enclosures just in case. I don't think you're a bad snake owner and your babies are in good caring hands. Unfortunately these things do happen at times. One last thing, in the future, practice good quarantine habits.