Hi, so sorry to know of the problems you're facing which must be making life pretty difficult at present.
Morning I have anterior/bladder prolapse.(along with hypersonic pelvic floor, perineal collapse,rectal intussception)
I'm guessing you've already seen a colorectal surgeon for the intussusception diagnosis? It seems likely that that this condition, together with the slow transit you mention, are also having adverse effects on your bladder function, so yes I think your instincts here are probably right:-
I also have slow transit, so that might be contributing?
As you have pelvic floor dysfunction, if you haven't seen a specialist women's physio for pelvic floor assessment I would ask for one while waiting to see Urology.
You might also like to have a look at a recent thread called "Constipation" partly for some links I posted (pages 2&3) on pelvic floor dysfunction & the associated different types of pelvic organ prolapse. One of the links has a section on the pessaries you mention, but the info in them is much more comprehensive than that so I hope you might find something helpful there. There is also a good book called The Pelvic Floor Bible by Jane Simpson. I'll bump the thread for you.
Wx