Nick has to have been involved in something, or why did he lie about where he was the night of Leoni's murder, which they also arrested him for?
Roz has to have killed Ifield, as her DNA was on everything and she had the fingers and hardware (did they find all that in the end? Need to rewatch), and the file with the passwords in on the phone makes sense, I have one though it's not called 'passwords' and would hopefully not easily be found! .
I assume Roz tried to fit Nick up for the murder of Ifield because she believed he murdered Leoni (and others?) and if he was going down for that he may as well go down for her crime - hence her comment 'our kids need one parent', she hoped he'd go down for both rather than her go down for one and him go down for the other.
Her explanation on why she didn't report Ifield's death was weak. Also, cuts both ways, why didn't he report 'hers' - although she wasn't dead, he thought she was. I think we're supposed to just think he was a bit of a freaky single guy who sleeps with prostitutes, hoards forensic evidence and has a thing about cutting up bodies.
It should have been a longer episode really, a lot suddenly got tied up out of nowhere which would have been better being explained. They threw Steve down the stairs just to prevent them getting too close to the truth - the truth about Nick, which must be about something else, one of the other murders, Leoni maybe.
I can totally buy that there are numerous BM all run by a gang, the BM are just muscle for hire.