Hi Glad you got some sleep! Really hope the nappy rash improves soon - poor little thing!
Forecful/fast letdown can cause poos like that, as you know, so maybe posture feeding will help. Also, when my DS had frothy green poos (and was quite unsettled etc) for a while, the doctor suggested expressing off some foremilk before feeds to give his gut a break from so much lactose. This cleared it up almost straight away. Do you express past the 'worst' of the letdown before feeding - it's something I've seen recommended.
Intolerance to dairy or wheat proteins can also cause problems, but I think that's fairly rare.

I don't know if you've ruled out reflux, but symptoms of silent reflux include swallowing lots and snuffles/difficult breathing (as it can irritate nasal passage etc). DS used to get this but appears to have grown out of it.

Good luck