Yes, if born at 31wks baby will be put in SCBU, though it may not be for as long as you think. Lungs are their no1 concern, followed by infection and feeding (babies don't have a suck reflex developed at 32wks). You'll probably be given steroids to develop baby's lungs (consisting of 2 injections, I know there's a set time apart, I think for me it was 12h, not sure waht happens if LOs born before then, I assume steroids are administered direct to LO thoguh can't tell you how). They'll monitor you, and yes, will try and stop contractions if they feel they have to, though despite my contractions, my waters hadn't gone, so they didn't try with me.
No nice water birth I'm afraid, but if it's any comfort, premes are usally 'easy' to birth (apparently - MW told this to try and give comfort "they just slip out"), if they don't section because LO's distressed (which LO might not be).
Hopefully someone that's been through it all will be along.
I know the lady in the bed next to me's waters had gone and she was having infrequent bouts of contractions but was allowed home (before me!). She had a UTI, so, well, just because all of the above MIGHT happen, it doens't always mean a preme baby. I listened to what they were telling her about SCBU. If waters do go, or there's any evidence of infection, be prepared to take antibiotics, and possibly a "large" amount for a long time (I was on them for 14days).
Other than that, it will totally depend on what's happening with you and your LO. I knew my LO was staying put, despite the steroids and not wanting to let me out of delivery until my heartrate, LO's heartrate went down and the contractions were less strong.
Please don't be worried to call an ambulance if the pains/tightenings become rhythmical or worse and you're worried. DH and I tried to get to hospital ourselves, but I was in so much pain from the UTI, that wasn't going to happen as DH was so distracted and the roads were so busy that, well, it wasn't worth it. I was much better of in an ambulance where DH knew I was being cared for, and he could slighlty more calmly make his way to hospital knowing I was as good as could be.
I'm sure it will be fine, and like you say, one almighty fart/bowel movement will see you right, but if at all concerned, get thee to hospital.