Hi bubblebell, I had SPD as severe as yours, I was in a wheelchair for 6 months with it and I well remember how painful it was. You have my sympathy for that.
I had a natural birth with ds1. In my case it was complicated because I had severe pre-eclampsia, diabetes and polyhydramnios as well. I was induced at 35 weeks and although I had a very long and painful labour (again, there were factors other than the SPD) I didn't end up with a CS.
My advice would be to talk to the midwives/doctors who will be overseeing your delivery NOW, and get them to draw up a plan in your notes for giving birth with severe SPD. I was made to labour on my back for over 24 hours without getting up, which is the worst thing you can do for a woman with severe SPD. You should make it clear that you wish to be allowed to try different positions and if for any reason you must deliver lying on a bed, try lying on you left side instead of on your back. I didn't know any of this when I had ds1, but after the traumatic delivery I did quite a bit of reading (on the Royal College Of Midwives site, among others) and learned a lot about how SPD affects delivery.
Talk to the midwives, as soon as possible. It might also be a good idea to ask to see a physiotherapy and ask her to put a short report in your maternity notes advising that the staff who oversee your delivery should take your SPD into account. I know it sounds a faff, but I encountered several midwives who hadn't heard of SPD, which I find pretty disgraceful.
Good luck!