He had her perfectly trained. The scenes with Anthony were hot and unapologetic. Luckily depending on choices mark may have that asshole streak so all is not lost.
Well, it is a bit up to Marc, and a lot up to Myriam really. Marc may become, or continue to be (depending on past choices) dominant (including the rough, pain, exhibition, deep throath, anal, etc. kinks of Myriam), even without becoming another Anthony. Or he can become another Anthony. Or he can be more or less submissive, or have no sexual interaction whatsoever with Myriam.
I don't think Zorlun will force the player (and Myriam) to chose only between the option of having Marc another identical Anthony, or of having him kicked out of the picture. Leaving only those to options, would +/- waste (big) part of the work he did till now on the character, and limit a lot the game in future.
For practical reasons, he will for sure need to limit the quantity of possible endings, but my guess is that if he has to limit a lot the number, he will try to make them kind of more complex, i.e. ending that show the situation and future evolution of the interaction with different characters.
Thinking program, one way could be to do by preparing different parts of ending, describing the situation and future interactions with the different characters and in the different environments based on specific variables, and then presenting them combined in sequence depending on the value of the variables.
It would need a good playthrough to check that the combinations (or using the URM files to change the variables on the fly for testing ;-)) of different pieces of ending do not become an incoherent mess, but it could allow to present a good number of endings and capture the complexity of interaction with various characters and in various environments (especially if one tries to leave open as many paths as possible for as long as posisble), without the need to create a big number of endings one by one.