I whole heartedly disagree with your evaluation of Deb, The MC spent 5 years in prison, having only been serving about 9 months when his wife leaves in hopes to get him to come back to his senses. What was Deb supposed to do? Tell him while he still had 3.5 - 4 years left in prison? Hope he calmed down enough by then to decide not to take another shot at trying to break the Official Secrets Act and likely get himself 25 - life (the whole reason the plan for the wife to leave in the first place)?
By the time MC got out of prison and you mentioned about him being suicidal going on benders, Deb has spent years wracked with guilt over a secret that she wanted no part of and again because he was still in prison likely saw no way to realistically tell him. Would not surprise me if she was almost as unstable as the MC at that point and only got worse as the 3 years between MC getting out and events of the story went on. The Mc's stubbornness and not wanting to hear anything she had to say could have just as easily pushed her to suicide (and very well may have if she did not get arrested after she got kicked out during the game) at any point in those 3 years if the truth got out because the MC was everything to her.
The event that started the down hill spiral was MC finding a picture of him and his Mom? (Milly calls her grandma but the woman has grayer hair than when the MC's mom is introduced) and he even talks about how his Aunt? (they used the name Mae but not sure who that was referring to as I forget if that is the aunt) was there as well. Despite this (due to Deb being the one there) he immediately loses it and forces out of Deb that she knew about Marie and Milly despite finding out later that Deb was not even at the event the picture was taken. Earlier scenes of Deb make it look like she is is very depressed at times, likely knowing the secret is going to get out soon, at this point her mental state is probably pretty bad. The mom then shows up after Deb is arrested claiming she is going to make everything right, tells the MC everything and how Deb was always against it, even saying that he should not blame Deb at all, the MC is super calm at this point despite finding out that the real people he should hate are his Mom and Marie. Faye (his mom) then proceeds to leave again before anything is really sorted out forcing Deb to again be the focus of Mc and Milly's hate.
The father is not really a factor at any point sure he plays a part in the deception but he is already long hated by the MC. He pretty much gives up everything in his career to allow his son to spend the 5 years in prison in a fairly low security facility despite at that point not talking to the MC for something in the neighborhood of 10 years. I would guess that the MC and Deb were like 18 - 20 when the Father caught them having sex (despite knowing about it) and kicked the MC out. Later a scene in prison shows Milly is like 10 - 12 and MC sees his father and does not even let him say anything.
After going trough the last few patches again I really feel like there is pretty much nothing to blame Deb for, by the time the MC is out of prison she feels so guilty about hiding something she never wanted to be a part of from the only man that she loves that there is no way she is going to be the one to tell him, the consequences would break her. Deb knows what will happen when he finds out, she has little left in her life beside the MC and knows he will never listen to anything she might have to say, yet still she is willing to help him and Milly reconnect knowing with Marie dead it is only a matter of time before the truth gets out. His ex and Mom are entirely to blame for letting it get to this point, the Mom most of all. Marie has moved on, she caught Milly masturbating thinking about daddy years ago and decided she did not want that for her daughter, for some reason despite knowing about the MC and Deb the whole time she did not talk to MC's mother about it and I would guess she would have learned something from Faye and maybe change her mind. The Mc's mom (Faye) seemingly went 3 years not pressing Marie about why she was hesitant or was afraid how her son might take it so she was fine to just letting it go despite knowing that her daughter was not taking it well.
The MC is to blame as well for everything turning out as it did, he is so stubborn that when he sees the picture he storms off and probably already has found Deb guilty of the crime before she admits it, despite seconds before that calmly explaining to Milly that his Mom and Aunt were both there when the picture was taken. He gets mad at his mom when she admits that it was all her and Marie's idea, calmly listens as she says Deb never wanted any part of it and that if anyone was to blame it is was his Mom. He then proceeds to go right back to blaming Deb despite pretty much giving his mom a slight hate free pass after she admitting everything was on her. The mom should never have left after that, maybe she could have made a difference, because it seems pretty clear to me that Deb and the MC are to unstable after finding out everything for her to have left.
