A spouse cannot be compelled to testify against a spouse....
en.wikipedia.org/wiki/Spousal_privilege
Might this be why she had him arrested?
My theory:
Roz was set up to frame Farmer, she believed he was guilty and 'powers that be' were ensuring she only saw the evidence she 'needed' to (as Hilton said 'there are facts, and then there's the truth' the implication being 'don't let facts stop you seeing the truth - the truth that I know is the truth').
She was all over this until Ifield got in her head. But she didn't know he'd gone to AC12, just that he was mithering at her.
She went to see him to talk it over and got annoyed with him, minor scuffle where she is knocked out, briefly.
His behaviour at this point is inexplicable though!
She wakes up, they have a brief scuffle, he gets knocked out - she calls Jimmy who calls the heavies, who come and clear up, she takes the fingers, phone and laptop but is confident no-one knows she was involved (why did no-one see the others arrive or hear anything?).
She is re-convinced that Farmer is guilty and the ID parade proves it (though in reality he's just a vulnerable young adult with learning difficulties who pesters women because he has no social boundaries - he did not rape the other woman, he was persuaded to plead guilty by Jimmy - yay for nana!) - she is prepared to overlook other evidence to see 'the truth'.
The prostitutes were all used by Hilton who is now being blackmailed by the gang that killed them (who knew they had been used by him so they killed them to use that info). Why they would suddenly bury a new body part is odd - unless they have panicked for some reason? Or they now want Farmer off and are thinking Roz or Nick make better fall guys?
Nick is innocent.