Abby has said, MANY times she and Lily aren't in a relationship. No cheating there.
Frank knows Lily hooks up with Abby and is fine with it. No cheating there.
Yes, though I'm referring to the situation where Frank had actively spoken to Abby about Lilly's feelings for her, which must have been sometime after the hotel stay. The first, not the version CNG uploaded later because his supporters went on the barricades. I don't remember exactly if Abby did consider the Lilly thing after the conversation, in the original version, but the whole post ultimately refers to that original version of an older update and how things could have evolved from there, which is no longer the case as that version was changed.
And even if it's not cheating, I think certain things are or were questionable:
Frank giving Lilly tips on what to do when she's intimate with Abby, with her mom as Frank's reference.
Frank and Lilly both know that Abby has romantic interest in Frank (quote Lilly: I don't trust her with you), apparently ignore it and don't tell Abby that they have something going on.
Abby and Lilly's rule for making out is that nothing happens between them if either of them has a boyfriend. Lilly hides her boyfriend, strictly speaking breaking this rule (again).
Maybe it's just me who sees it that way, but if I were Abby, I'd be pretty upset. At least as far as the two of them are concerned. I think Abby will have noticed Meiko's feelings for Frank and won't hold her secretiveness against her. Especially since she was more or less blackmailed by Becca.