As for conflict/struggle in their relationship, one direction is Marc's jealousy. Albeit a cliché, it exists on some level in a lot of relationships, it already reared its ugly head in Kay's apartment. And if Marc and Kay do decide to try it on, Kay will be so much more accustomed to the lifestyle - Marc would have a lot of learning and growing to do, I can see lots of friction there.
Kay has already been patient with Marc, but he'd have to know that he would be the "leader" (I hesitate to use that word, but he will be showing Marc a lot about something with which Marc has no experience), and allow for mistakes on both sides.
I forgot about the jealousy, but you are right. Another sign telling something about his feelings.
But after reading Vincent's comments, I guess he's right, realizing his feelings were deeper than he thought, is a long way from accepting and taking steps.
I think that could be the struggle. So I re-write my part 2: Marc doesn't know anything, who he is, what he wants, what he feels. He's not ready to contact Kay, doesn't even know if he wants to. But there are the genius flashbacks (we see a lot of Kay in part two thanks to the flashbacks). I don't know if there will be much conversation in the flashbacks (their feelings were palpable without much conversation, that was crucial for the film) but I’m sure there were are lot of nice/good/intensive moments between them we haven’t seen so far.
And these flashbacks help Marc to sort out his feelings.
(Do you recognize this: hearing a song or being in a certain place that brings memory’s back. And these memories make you realize things).
The end of part 2 could be open again. Marc is ready to contact Kay en so he does. What happens next we can see in part 3