This is essentially a reverse corruption game. In Dating My Daughter, Daddy is in control and is slowly corrupting innocent Dee who can barely make her own decisions, and there are plenty of other similar games with that basic premise. In this game, the daughters are the ones doing the corrupting and pushing boundaries. Frank is in the role of Dee, not quite as helpless, but still, he is not the one setting up the situations, the girls are. Frank is the innocent, the girls are the deviant, always pushing and prodding Frank out of his comfort zone. I think that twist makes this game stand out of all the other incest corruption games in a good way.
I agree with you for the most part, but would add one more thing:
The act of corruption emanates largely from Becca and to a slightly lesser extent from Abby. Yes, Frank is the target of the corruption, Lilli however I think is also manipulated by Becca into finally opening up to Frank (in more ways than one).
Examples:
Becca pushes Lilli into Frank's arms so that she virtually hugs him.
Becca is mean to Lilli, on the one hand to consolidate her own place in the pecking order and on the other hand so that Frank defends Lilli and she realizes: Oh, Frank, the man to whom I was always mean and dismissive defends me.
Becca clearly folds Lilli up nastily so that it gets into Lilli's head that Frank has been a father to her longer than her biological father was.
The game at the pool visit where the loser has to bare in front of the winners in the sauna and Frank who says nice things about her body (which she is known to be very insecure about).
In the evening, Lilli is made to dance with Frank while all four are at home.
Also, we don't know how far or if at all Becca manipulated the breakup of Lilli and her ex, but since we know she has a plan she's been working on for 5 years it will probably have been water on her mill wheels that Frank had to tell Abby right then that she wasn't his biological daughter, since Lilli had to take care of Abby like that (and she knows how close Lilli and Abby are) consequently it wasn't surprising that her future ex had to lose out in favor of Abby.
How far Becca's manipulations really go we will probably only find out with Becca's side story or in the later course of the plot (which I call the Becca section).
The sections look like this to me:
Lilli section: important characters are introduced, divorce is finalized, Lilli's behavior towards Frank is slowly softened and explained, Abby learns that she is not Frank's biological daughter and begins her own plans.
Abby section: Lilli's self-worth is slowly built up, Abby slowly breaks down Frank's resistance, Abby confronts her birth father, the situation surrounding Vanessa and Beth is brought more into the plot.
Becca section: escalation with Shannon and her lawyer. At the latest here a decision around the situation with Beth and her parents' wishes, confrontation with Lilli's ex and with Shannon (through Abby, whose personality becomes more like her mother), the exact extent of Becca's plans are revealed.
Also, I hope the dynamic between Vanessa and Frank stays as it currently is. I like their synergy as it currently is and anything beyond that would give it a stale flavor.
In addition, I've been thinking about how each of the Core LIs would react to the "Frank getting Beth pregnant" situation.
We already know from Abby that she would love the idea of having a little sibling. No problemo.
I don't see much of a problem with Becca either. I think she would only have problems if she was left out.
With Lilli, however, it depends on when she finds out: Too early and she will probably have a short-circuit reaction again. In her nightmare (in her side story) we have already seen that she sees herself as the unloved middle child (although I assume that Shannon has a dislike for all three) and it can be assumed that if a woman like Beth were to expect a child from Frank, that she would react with fear of loss, or that she would lose her situation with Frank, even as a father, against a woman and a baby.