The thing is, the fighting had to have started after Stolas had the affair. Do you think Stella would have pretended for 17 years that she actually cared for Stolas? The Stella that pretty much reacts to the extreme, before she even thinks through all the consequences? This Stella wears her emotions on her sleeve...and is an idiot.
This is why when people are saying "Oh that evil smile Stella has, is because she's so cunning and manipulating Octavia to be on her side". Bitch, this woman literally screamed that she wanted her husband killed at the dinner table right in front of her husband and daughter. The only reason why they didn't hear her was because Octavia has her headphones in, and Stolas was too engrossed in his imp erotica (seriously dude, could you not have waited until you got into the bathtub? You had to read that shit during the family dinner?)
Stella would have probably been throwing a party for her husband's downfall, but instead she actively goes looking for her daughter and offers her comfort. Not for some grand scheme has, but because she understands that this is Octavia's father, and of course she would be upset by this whole thing. She can absolutely despise Stolas, while still being caring towards her daughter. She can do both.
Tbf, Stella did throw Galas dedicated to shitting on Stolas. What was that one party named? "Not Divorced: Anniversary Party! Couples only!" along with her knowingly badmouthing him to her friends within earshot. She didn't exactly pretend to care about Stolas unless relationships among Hell's nobility are genuinely that awful on average and not being divorced yet is really that much a cause for celebration. Not to mention, despite being unhappy about her marriage, she never engaged in an affair, even with her peers.
EDIT: As an aside, I really want to read a fanfic of Mastermind, but the crime I.M.P. is accused of by Andrealphus and *audience gasps* Octavia is the murder of Stella. An AU of The Alternate Deal or whatever it was called with plenty of details changed and conflicts for non-main characters.
Like, imagine for a moment if Vortex was one of the investigators. As a Hellhound, he will recognize the smells at the murder scene right away, Beel will likely plead with him not to reveal the truth because Blitzo and Loona are their buddies.
Crimson isn't going to be called on a witness at all, he'll be too busy throwing a party over a Goetia being dead.
Who knows how Asmodeus will react like. He's opposed to rape, so odds are he takes a dim view too on murdering someone for being in the way of a relationship.
Or take the climax of the original Mastermind and warp it. Make it so Stolas rushes to the trial, puts on his show as The Mastermind (tm), only to trail off when he sees his daughter as one of the prosecuters, realizing he just ruined her image of him. Obviously, it's going to end with the execution of Blitzo and Stolas, but the fallout from this shitshow is going to need a second chapter.