Izchaz, I have just read all your posts on this thread - and am so sorry to hear what a terrible and difficult time you've been having. It sounds traumatic.
You are very articulate and, the more you have posted, the more a picture is built up:
i) Your deranged and sociopathic 'friend' completely did you and your H over, wreaked total havoc between you and him, using your H's weakness and ability to be manipulated to set him up to do this.
ii) The whole thing has blown up and, as well as blaming the friend, and even you for foisting the friend on to him, he takes responsibility for his part and is remorseful, and is still beating himself up, that he allowed himself to be manipulated into doing something so against his nature, something which hurt the woman he loves. Both of you are angry, upset and traumatised about different aspects of this, and have unresolved, strong residual feelings about what happened.
iii) After initial reactions and separation and fury etc., you have made the choice to try to recover from it, work through it and repair the marriage.
But, for the reasons above, when you argue about other stuff, all the unresolved feelings leap to the surface and invade/escalate your argument so that it is a full-on, vicious, haranguing 6-hour row, which neither of you can put a stop to, because your emotions and feelings, the anger you must feel, the disappointment he must feel in himself, how abused/stupid/sad/upset/shocked you feel about first being treated like this by the 'friend' who you were trying to help, but who was playing you, and the feelings about the enormous thing your H has done, are all so raw that they cannot be kept under the surface permanently.
So your marriage is suffering as a direct result of not having been able to recover from this terrible, and actually quite recent and long-drawn out, complicated event.
The only thing I can think of, if you BOTH EQUALLY want to repair your relationship and are prepared to actually take action (him, I mean, because you evidently are), is individual counselling for you, and the same for him. Try to get a very good counsellor/therapist on recommendation, SO important.
Because this trauma and all the complicated feelings around it is something that you are still suffering over, even if what you are actually arguing about is other, more trivial stuff.
Perhaps you could agree that the problems you are having require you to undertake the drastic action to live separately for now, and each attend counselling separately to work through your own stuff, and he his, but with the intention/hope of coming together again and perhaps, at that stage, doing some therapy together, perhaps CBT to address the way you now live together, and how you move on.
This is a HUGE and SERIOUS thing which has happened, with a PROFOUND effect, and it will have to be a HUGE and SERIOUS thing you both do to get past it. DRASTIC ACTION IS NEEDED. It cannot be swept under the carpet.
I wish you the very best of luck.