I think it was Jim. He has a pick-up truck, which puts him in the frame for the first two attacks, and his "why would I rape her when I'd had consensual sex with her that morning" line ties in neatly with the dispelling of other commonly-held misconceptions about rape that the series has already touched upon. Cath has a hold over him (does she know something about the earlier attacks?) and now they've coincidentally decided to leave Broadchurch (his suggestion, I think?).
I don't think there's an underground porn circle or sex gang; I do think we're being shown some of the effects inside a small town of rape culture and all that goes alongside it.
Ed is obsessive and has a violent past: doesn't make him a rapist. Ian wanted to spy on Trish because he suspected she was having fun when he was supposed to be the one doing that after they split and he found himself a younger model, so he got CreepyTwineBoy, whom he used to teach (which is why he knows he's an IT whizz) to install some spyware on her laptop. After the rape happened both of them panicked. but I don't think there's anything more sinister than that going on (although that is quite sinister, obviously).
Taxi man and his wife and the son have an untold story which will come out, but I don't think it's connected to the rape. The son and Tom Miller are friends, so maybe that story will connect up with Hardy's daughter and the photo, and the upshot will be that taxi man is a wrong'un, but again not a (or not this) rapist.
Can't work out, though, why IceCreamGirl lied for CreepyTwineBoy. Does he have any connection to Jim? Or was he doing something else entirely - maybe connected with Ian?
Not as good as S1 but I'm still really enjoying it. Some super acting all round, I think.