Is there anything in the code yet that ties the Sam storyline to the proposed partner swapping with the young couple? ie if Sophia agrees to become Sam's Secret Girlfriend, she won't agree to the partner swap? If any two storylines could impact each other, it should be these two.
BTW, when is Liam's next appearance? Their next scene together, or shortly thereafter, should include a discussion about whether or not to swing with the young couple.
One possibility for a proper appearance of Liam is of course the visit of Sophia's parents.
It is important for Sophia to prove that she is also a good musician/cellist.
It is important for Sam to introduce himself as a future son-in-law.
We may learn something about Sophia's past and Patricia's obsession with her sister.
Maybe Sophia's mother realizes that Sophia and Sam have something more in common than a teacher/student relationship. And she approaches Sophia about it. To which Sophia, out of embarrassment/apology and distraction, tells her that Liam suggested a swinger's night or a more open marriage.
To Sophia's surprise, her mother says that she and Sophia's father have been in this kind of relationship for a long time.
This ultimately triggers her to talk to Liam in bed that night and suggest a second evening with Cathy and Andrew - "Let's see where this goes!