I think that if Lena defended her independence and refused, then Seymour will try to get close to her through Ian who works with him. But if he is romantically involved with Lena, he may be able to build a partnership with Seymour, showing his qualities and strengths that will come in handy in Seymour's business and somehow not give him too many opportunities to influence the situation with Lena. I see Ian as the administrator of one of Seymour's establishments, sort of a "negotiator" for him (for example, with old perverts like Agnes or something less corrupted) or doing something in Hierofant.
That sounds like a plausible way for it to happen. Though it would still limit Ian to being a pawn in Seymour's game, at least in the beginning. And is it likely that Seymour will ever admit defeat in his pursuit of Lena, as long as Ian has a relation to her? Not necessarily.
I guess the alternative is that Seymour really tries to destroy Lena's life, as he threatened to — and that he somehow also tries to get Ian to help him. Either through being open about what he's doing, or through pretending to have good intentions, and somehow tricking Ian into helping him without understanding the consequences of what he's doing.
Let's say Ian and Lena decides to be in a relationship, and Lena then sleeps with Axel immediatelly afterwards, while she rejects Seymour. If Ian finds out about Lena and Axel, and Seymour then offers him lots of money to help him make Lena's life uncomfortable, it would give him a reason to choose Seymour's side, with Lena being on the other side. It would be petty of him, but still.
Generally I like the idea of Ian and Lena ending up as kind of enemies, through how they decide to relate to Seymour. And considering they both had the chance to cheat immediately after the relationship talk, I get the feeling there will be chances for their relationship to end incredibly bad way before the end of the story.
And yes, as several others have noted, Seymour is an intolerable, self-righteous asshole. I don't like him as a person at all. But I absolutely think he's good for the story (or at least he could be, it's still a bit early to pass a final judgement).
Edit: And regarding the picture in Eva's latest Patreon post: The file name is actually "v4_seymour4.png", so I guess that removes any remaining doubt about what scene it's taken from.
