Seven hours is scarily fast. Someone could see their bunny eat their breakfast happily, go to work/college etc and the bunny be gone by the time they get home
I lost Izzy at about 8/9 months old she stopped eating and "didn't seem right". She died in a little over 24 hours even that seemed scarily quick. She died of a twisted liver, I think Mable probably had something 'go wrong' that was totally unpredictable/unpreventable too.
You did a brilliant job nursing her, I'm sure if she had had a simple digestive upset your nursing would have pulled her round.