Thank you guys.
The post mortem showed that she didn't have anything contagious that would have caused the egg binding and subsequent prolapse - all GI swabs were fine.
She did have another egg at the top of her follicle that was already being formed. So the vet thinks she was looking ready to start a full fledged egg laying spree.
Her cloaca had some haemorrhages and some tears in it. This caused infection. She had lost a lot of blood the night before from the prolapse and this all contributed to her little body giving in and not making it. The most likely cause for her prolapse is the spinal damage that put pressure in places it shouldn't have and probably made her strain to lay the egg.
The vet also found that Bubblegum was unusual because instead of the normal one developed ovary and uterus that all birds have... Bubblegum had two ovaries and two uteruses. Very strange but it is unlikely that this would have been the cause of the prolapse.
My vet saved some feathers from Bubblegum for me which I will pick up as soon as I can.