I've worked with fabrics all my adult life. What I see in the condition of the quilt is damage to cellulose fibre caused by humidity rot. I'll do my best to explain:
Cotton is around 90% cellulose and cellulose is hydrophilic - basically it loves water and absorbs moisture, unlike synthetic fibres that have a 'wicking' effect when worn (this is why synthetic fibres are best for ski and sportswear, as they do not stay wet/hold moisture).
What I'm seeing in the state of the quilt are cotton fibres that absorbed moisture/liquids from the body and then were placed in a humid, completely unventilated place such as a plastic bin bag. With no evaporation the cellulose (cotton) fibres rotted and the synthetic fibres such as the polyester hollow fibre batting, remained largely as they were. I think this is why the pumpkin octagon remains this way, because it contains no cellulose - if you look closely the weave is a dense twill that is different to the rest of the identifiable patterned parts of the quilt. (Even if cotton, this kind of weave would take longer to break down than the normal weave of cotton quilting fabric). The polyester thread is very strong and has not broken down. (IMO, a quilter who took pride in their work would not have used polyester, but would have used cotton thread. This is because polyester thread actually micro-damages the cotton fibres - when stitching cotton fabric use cotton thread, when stitching synthetic fabric use synthetic thread. This is a budget amateur quilt, IMO)
If you look at the clothing too, you can see how the garments were scrunched up or folded when stored - the rotting cellulose from the cotton has gathered in the creases/folds of the non-cellulose (synthetic) garments - see the centre back of the pink coat. This, to me, is not the pink coat rotting, but rot from a cotton item - this could be from the quilt, or the very badly rotted cotton 'smile' tee shirt.
The items that I would identify as cotton from images I've seen are: black smile tee shirt, top part of tutu dress (mixed in with elastase/Lycra, hence it's holding together somewhat), and the quilt. The shorts are, IMO, synthetic, as are the pink joggers that I think are a heavyweight interlock polyester.
In brief, I think that the near total disintegration of the cotton parts of the quilt are because the body was stored with it, in a sealed humid place, somewhere like in plastic a hot loft, or in a stored caravan - the body fluids, humidity and lack of evaporation would cause the rot of the cotton fibres. The other clothing, if not actually in contact with the body, would have been stored similarly, with moisture, humidity and lack of evaporation being a factor, though I do not believe that the clothing or body were stored in water.
No links. All this is my own opinion, based on my viewing of the images of the quilt and clothing that are in the public domain, and my decades of experience with fabrics and fibres.
Sorry for the ramble.