Ok, so for the moment no sharing/cuckold action then
At this stage, it's not clear whether they feel the same way about each other. Because so far Nadia hasn't thought about Eugene, and he hasn't thought about her. I expect after a while, they both just moved on, because they never expected to meet up again, or considered it unlikely. But we know they will meet up again eventually, and since they share a daughter, they have a good reason to resurrect their relationship, at least in some capacity.
So maybe they will end up together again? But this time the cucking will happen ( if it does), because Nadia is already involved with other people. So I don't think it's likely to be the same as it was in Part 2, where she was basically just cheating on him. Maybe as they're starting to renew their relationship, Eugene will become aware that Nadia is already involved with other people, and this will make him jealous, realising he's no longer the main man in her life. I don't think the NTR will be the same, and it certainly won't be as brutal, because both him and Nadia have changed so much, and Nadia in particular is likely to feel that she owes Eugene big time.
For most of Part 1, Eugene was pretty much a loser. So the game had a very different dynamic to Part 2. At the end of Part 1 though, Eugene did a very noble thing, by sacrificing himself to protect Nadia. So in that moment, he went from zero to hero, and it's much harder now to justify him being cucked, even if he's sometimes very submissive and seems to get turned on by it. During most of Part 1, I didn't care about Eugene, but now I do, because when it really mattered he proved that he was a real man.