Back-to-back labours do tend to be longer as baby doesn't usually descend as quickly and if he/she decides to turn on the way then that can take time too.
Toby was back to back and not engaged at the start of labour so it did take him a looong time to get moving, i found that sitting on my birthing ball leaning forwards over the side of the hospital bed really eased the back pain. Other than that i have no comparison to how a 'normal' labour would feel so can't comment, but i did keep feeling a lot of pressure in my bum which would make me want to raise my bum up in the air during the peak of a contraction
I did have an epidural later on in my labour with him, although not because of the back labour but because of the synto (hormone drip to speed things along- which wasn't given because he was back to back but because my waters had broken and i was group b strep positive and my contractions, although strong, were quite irregular)
I don't think spd should make a massive difference to either you or LO during labour, although research suggests actually an epidural is not such a good idea if you have spd as you may then be encouraged to open your legs wider than you can actually manage with the spd (iykwim?) or adopt uncomfortable positions when you have little feeling, and then end up hurting yourself without realising.
Water (i.e. a bath in early labour or using the pool for labour/birth) should really help with the pain from the spd and give you more freedom of movement and buoyancy. I had mild pgp with Toby but quite severe pelvic pain this time to the extent i can't really walk properly and i know when i go to aquababies with Toby every week the feeling of getting in the warm water is wonderful, i'd like to stay in there forever

so i'm hoping labouring in water will have the same effect.
There are a few things you can try to get baby to tip so his spine is towards your abdomen, spending plenty of time on your birthing ball, or on all fours for e.g. and not too much time reclining backwards or sitting in a car seat. Ideally you want your knees to be lower than your hips as much as possible.
This LO was back to back for a while earlier on but seems to be behaving at the minute (for now anyway!) i have spent a lot of time on my birthing ball rotating my hips like i am doing hula-hooping

Sometimes they just get comfy a certain way though and there's nothing you can do about it
Back-to-back labour is one of those things which can make a big difference to how labour progresses, in the sense it can definitely slow things down and make it more intense, but equally, there are lots of women who give birth to babies who are back to back and come out facing up, without even realising! I think sometimes it has a bad press iykwim?
HTH and helps to put your mind at ease
