The second scenario wouldn't work. Because Katherine would just simply make him stop. As Myriam has done every time, if you allow her too. Cassius only manages to get his way, because Myriam tends to be very submissive, and thus she gives way very easily. But Katherine isn't a very submissive person, and therefore Cassius wouldn't get his way.
There's only one realistic way, that Myriam and Katherine could have sex together, and that's if they have personality transplants. They'd have to become completely different people
How much screentime has Kat actually got? What can we confidently say about her personality? Based on what we've actually seen, we can pretty much only say that she's sexually inexperienced (even her sexuality is up for the player to choose). How would she actually react if she were put into that situation? I think there's plenty of room for things to go in a lot of different directions.
(But don't think about it too hard, it's just my bad fanfiction that I wrote.)
I don't think it's realistic to expect a woman who was completely submissive to her boyfriend for years, to suddenly transform into a dominant character in just a few months, if at all. I think there's plenty of evidence that Myriam is becoming more self confident and unwilling to be just a doormat anymore. So I disagree, I think he will use it eventually, it's just not going to happen overnight.
I don't mean to be condescending here, but here's a very basic thing about how stories work: Audiences come into a story expecting to be delivered promises that the writer makes to them. Some of these promises are delivered directly by advertising, some of which are delivered by events in the story itself, and some of which the audience brings with them due to genre expectations.
This is why so many people ask about when there's gonna be Katherine content and are surprised when the answer is "no", because the erotica genre, especially WEGs, have the basic audience expectation that if a character related to the protagonist is introduced on screen, there's the implicit promise that there's gonna be incest with that character later. You'll notice that in games where there isn't gonna be any incest, the main character's family is usually nowhere to be seen. And it's not like incest or lesbian content is off the table in this game either.
And in games, some of those promises are delivered by the mechanics and the UI. In this case, if there's a stat the game tracks as you make choices, that implicitly promises to the player "Hey, this number is gonna be important, there's gonna be a choice later where you need this number to be high enough to get to make it."
Zorlun has talked about this in the past and says their philosophy is to add every stat they can think of, in case they think of a use for that stat later, without having to go back and modify earlier parts of the game (and force to to start the game over). And that's understandable, especially while the game is in an incomplete state, but it *is* very weird there's all these affection stats for characters who show up once and are never heard from again, as well as kink and personality stats that don't actually do anything.
tl;dr My response to your point of "She's been a submissive her entire life, why would she change personalities" is "The existence of the Domination stat implies Myriam has the potential to become a femdom."