I had a 'frenemy' like this. I've used the same word as you - horror - to describe the period in which I acknowledged what she really is. That period was the beginning of a shortish, unbelievably painful, tearing of veils from my eyes regarding my husband, boss, colleagues and family members. It did cause me a breakdown; I was the one who ended up in a mental hospital. "Truth hurts" has resonance for me, though not in the way it's usually meant.
Of course it takes a minimum of two to make a relationship, toxic or healthy. My part in my own downfall? Blind trust and irrational optimism ... heroic efforts to see my world as I wished it were; denial of the first order. Just like yours.
Your ex-friend can't have been successful every time she tried. The thirteen instances you & her H have identified are those where she succeeded. The others? Her 'failures' didn't deny to themselves what they were doing with her, or she with them. Unlike them, your husband abused a woman's apparent vulnerability to stick his dick in her. This makes him a nasty person, a sexual predator. He went on to lie to you, abusing your trust. This makes him a dishonest person, unworthy of trust. He blames a woman for making him have sex with her. This makes him a fucking chancer, I'll give him that! And he blames you for minding that he lied, cheated and abused. This is classic abuse.
"She made me do it" 
"You made me do it" 
"You should be sorry" [no emoticon]
In the film, Single White Female, Hedy sneaks into Sam's bed and gives him oral sex. Sam refuses to keep it secret from Allie. Hedy kills him. I'd be curious to know whether your OW attacked your husband with a stiletto?! Did threats to his life make him get an erection, use it to have sex on a dangerous, pregnant woman, and abuse your trust? Would it not have been more rational to go to the police, or at the very least tell you and her husband of the threats?
" Last night we fought from 9 at night until 3am, and only stopped because our lodger came home. Once he has started he will follow me from room to room, verbally attacking and prickling me until I re-engage the fight."
That's bullying. You're married to a selfish, dishonest, unfaithful, manipulative bully. Well done.
Marriage isn't a shackle, you know. You can stop being married to him.