Well, maybe Marc and Cassius could get an arrangement, and if Anthony was around, as well - but for sure William would not "share" Myriam, and be maybe even a bit uphold at the idea.
Myriam is his great romantic love, effectively, if she developed too much some potential future activities, it would have to be seen how that would be compatible with William's view of her (unless she manages to convince him to get in, there were case of porn actresses who started doing porn with their boyfriend, or even husband ).
For the "dominatrix", well, nothing is impossible, but considering the list of people you put, that would seem to require that she went for being dom with Marc, in control (which in practice, means cutting out half of the activity) with Cassius, probably nothing done with William, did not do a lot of stuff where clearly she lost control or was not in control (so that her "dominance" is much higher, or at least higher, than submisiveness), and go through further scenes where she pushes her dominance up.
Definitively not something that can happen easily, and in one single update, IMO.