Regarding the gelatinous sacs encasing kiwano seeds; biochemically speaking, what exactly are they made of and how are they formed?

They are referred to as mucilaginous and it is possible that few scientists have fully analysed the sacs.

Link

Certainly, there are a number of beneficial substances including vitamins and minerals:

Paper

If you cross-reference the paper, you may find out the information you need.

It's probably very much like any other sort of melon.

But it looks cool.

 

Realistically, like all plants they are mainly water, held together with things like cellulose, pectin and starch.
The mucilaginous bit is probably similar to other gums.
https://en.wikipedia.org/wiki/Natural_gum