So sorry to hear things didn't go well last night.
I have to say I agree with Tory. If you have to use 'sposies at night then go for it. It's more important that you BOTH get a good rest - sleep is so important, it's when the body release growth factors for healing, etc... so is vital for you both to recharge your batteries and stay well in yourselves. At the end of the day, what's one 'sposie a day compared to the number used by full time users (and I can attest to that as my DS has had to be in 'sposies for about the last 6 weeks!).
It does sound like it could be the wetness or the bulk waking her up, although I think if it was the bulk she'd have problems settling to sleep to even begin with. It sounds like the wetness is more the problem as she's woken up screaming when the nappy is sopping wet. Do you think her wee is burning her, and therefore at the root cause of the sores? I know you've had these sores for a while - have you spoken to a HV/GP? Maybe they could arrange a urine test?
Sorry, I bet you've been over all that ground, but thought I'd mention it just-in-case.
for you, and