Problem we have is that so much can, and will, change in the 5 whole episodes remaining!
Right, my latest thinking:
There are several interwoven sub-plot strands, to bamboozle us with overlapping red herrings. Mostly hingeing on RL illicit sex, and an online porn ring. Maybe covertly-taken, amateur footage, and some featuring Broadchurch locals?
Supplying the films to subscribers could well be Arrogant Rope Boy's money-making sideline; he has tech expertise and said "fishing net business was going down the toilet" to Hardy.
Think the teens' phone porn was not Trish's rape.
Trish bi, perhaps? Not an issue in many places, but more controversial in small community Broadchurch. Also probably not keen for her daughter to know.
Thus her (so far) refusal to disclose her Saturday morning lover (female) for Hardy & Miller.
My guess is that one (or both?) of the Atwoods have been having sexual fling with her. Either she was sleeping with one of them, and the other spouse found out...(Trish, though, remaining unaware of that discovery. Hence innocently attended party).
Or she was simultaneously sleeping with both Atwoods (separately!!), and each found out about their spouse...(Trish again unaware of the discovery and still attending party).
One took ballistic, vicious revenge on Trish, for her perceived 'cheating' (and abuse of friendship, if Cath)...attempting to frame their 'cheating' spouse in the process.
(Much more complicated for Cath to exact punishment by rape, but not unheard of: a woman being responsible would definitely be the last possible deduction anyone would come to, as well as really making viewers question any firmly-held views, which series 3 is doing so well.
Does make her so-called sympathy act towards Trish particularly grim, though, so unlikely 
).
I suppose if Trish was knocked unconscious, then concussed and traumatised afterwards, she might not have recognised the intimately-known familiarity of attacker, or dismissed it as impossible...