If Don was writing this story right now I could see your outline. But I strongly believe someone else Is writingYou know what I am thinking of a "trad" ending here
Dan and Lester have to unite to protect Sarah from someone ( say that stalker whose name I can't recall)
Lester gets a heart attack and a change of heart (figuratively) and decides he loves Sarah . Like "love
That becomes his downfall and sentinel starts catching up to him and now Dan has to protect him and participate in his schemes
Lester with his new morality corrects the couples life while he is dying meanwhile Sarah gives birth to his baby which is about sealing the trio forever
Lester soon dies and Dan and Sarah bury him and all saying he was their "good friend " .
Sarah still pines for him but she has a family and Dan to tend to and realizes her mistakes . Dan meanwhile is finally cured of her cuckolding kink and searches through Lesters stuff to finally discover all his tapes and shit.
He destroys them but keeps some for his own pleasure. He never tells Sarah out of his final respect for the monster
This is the only "trad" ending I can think of where all 3 get their own ways and move on
and the story of Dan and Sarah for me is over.