I did have that idea, that Seymour might be her father. But it doesn't seem likely to me. He'd have to be pretty nuts, to be putting his own daughter through this twisted sex game that he's playing with Lena. And could you really see a guy like Seymour with Lena's mom? Her and her Mom, don't appear to be very alike, so what would be the attraction for Seymour?
You say Seymour isn't a bad guy? Are we actually playing the same game? He's the biggest villain in Baluart. This is a guy, who is profitting on other people's misfortune, and is deliberately rigging the local economy, so housing prices and rents are pushed artificially high, wages are reduced and small businesses struggle to compete and are pushed into bankruptcy ( allowing vultures like him to swoop in, and buy them for ridiculously low prices). Plus, he's already admitted that he's fixing the Literary competition, so he can decide who wins. And it's obvious, that he's planning to interfere with the city election for Mayor, so the guy he controls gets elected. And you can bet, he'll then be planning to make things a lot worse, so he can profit even more.
And that's not even including what he's doing with Lena. If she refuses Seymour's offer, we get to see a very different Seymour. He makes it perfectly clear, that he will destroy her, her family and Ian's career as a writer, if she doesn't take up his offer. We find out he did threaten various people who were employing her or might employ her in the future, and that's why job offers suddenly seemed to dry up. He threatens to have her accounts on Social Media suspended, he also threatens to put Ed and Molly out of business, and take their business over, for a fraction of what it's worth. He comes across as a very nasty piece of work, who is prepared to do whatever it takes, however evil, to get what he wants. And this other side to his personality, is what he's hiding from Lena, if she does take up his offer. Everything Emma says about him is true.
Axel and Robert, are just minor villains compared to Seymour. Neither of them are clever enough to hide their true intentions. Plus, both do at least love her to some extent. They might be taking advantage of her, but both of them are motivated by love or lust. With Seymour, it's obvious his motivations are much more sinister. He's actively trying to corrupt Lena, and not just sexually.