I'm so sorry for your loss.
I think the only person who can really give you insight is whoever does the autopsy.
My suspicion would still be carbon monoxide. Gliders, like birds, are so small that they are sensitive to anything inhaled and wouldn't need the same level as a human to affect them. A small amount that might not affect a person probably could still affect a glider, from all I've read. Even the fumes from a heater would make me nervous.
I hope you get the answers you need. I'm sorry this happened to your fur babies.