Unfortunately for you, I don't think Seymour is planning to share Lena with anybody, in the long term. This is a guy, who basically wants to own everything, and Lena is going to be his ultimate possession. As he says during their first meeting:
'A man who possesses as much gold as his heart desires, ceases to appreciate it's shine. His eyes search for a rarer glint, a gem he has no possession of....yet'
He claims this is why he wants her to model for him, instead of a model who has already achieved success. But of course there's more to it than that. Seymour plans to mold her, into a partner who is worthy of him. Someone who he can ultimately 'pass his legacy' on to. So if your Ian is in a relationship with Lena, and Seymour is her patron, he's the last person you want Ian to be on friendly terms with. Because that means he has Ian by the balls, and one day he will use that against him, to take Lena away.
Seymour is probably going to be Ian's greatest adversary for Lena's affections. Forget Axel, Mike, Jeremy or Jack, they're just temporary distractions compared to Seymour. He is the one, that Ian will ultimately have to destroy, to hold on to Lena. Because you can bet he will try to destroy Ian, if he doesn't give way. So sorry, I don't think that friendship is going to last very long.
Why do think Seymour is being so nice to Ian and Lena? He's making them both dependant on him, so he can control them. Lena's entire modelling career, is now dependant on Seymour. Most people won't work with her, if he doesn't want it. He's got her off Stalkfap, which might have been a very lucrative alternative source of income. She's blacklisted from working as a waitress. He even tried to take over Molly and Ed's cafe. On top of that he's paying for her Mother's expensive medical treatment.
And then as far as Ian is concerned, he works for him, he basically controls the literary competition that Ian is entering, and can either fix it so Ian wins, or make sure he has no chance of winning. He can sack Ian at a moments notice, and make sure he'll never get a comparable job in Baluart. He can ruin his dreams of being a writer. And that's what he will do, if Ian tries to oppose his attempts to seduce Lena. He'll probably threaten to destroy Lena's career, stop paying for her mother's medical treatment and make her pay back what he's already paid out for it. Ian may have to find a way, to bring Seymour down. And my guess is, somehow he'll get evidence that Seymour has fixed, or is planning to fix, the City Mayor's election, and that is how he'll ultimately win.