I'd agree with camube that the voluntary working with Seymour is quite "believable" -- if Lena forms a positive first impression of him (which is easy enough, she kind of expresses it by default) i can vey well see her getting swept up by Seymour's charisma and rhetoric, combined with the "high class" treatment she receives (particularly if it's high posh
Lena who is more responsive to displays of wealth etc).
The player might roll their eyes at Nietzche being touted as the top dog of philosophy, but Lena, while potentially intelligent, is still a fairly regular person, one who can easily buy into this mindset (similar how many people come to believe Machiavelli was a political genius because he's such edgelord about it) and the idea she's interacting with top-class intellectual, and strive to match/impress him rather than laugh straight in his face.
I strongly disagree. Lena's supposedly read enough philosophy that she's familiar with Nietszche's three metamorphoses even before Seymore can tell her of the Child. So she, even more than most, should recognize what a ginormous red flag it is when Seymore opines that he prefers to think of "the Child" as "The Master." It not only completely invalidates the whole point of the original metaphor (rather than being reborn into a whole new world, he explicitly chains the supposed metamorph to the old world's hierarchy) but it makes it abundantly clear that he considers himself utterly beyond any sense of morality or rules.
Even beyond that the narration of that whole dinner is littered with Lena being unnerved by Seyemore's various turns of phrase or general predator vibe, to say nothing of his establishing character moment harassing the local homeless man. Or the tense music. The scene was clearly meant to drive home that Seymore is going to be the villain of the game, not paper over his faults in a way that might plausibly leave Lena unaware how Faustian Seymore's offer is. I agree Lena's supposed to be desperate enough to consider his offer even so, but that's just to fuel the corruption/exploitation kinks Seymore is supposed to embody.
If she'd actually had a decent working relationship with Seymore before the interview/dinner I might be willing to accept Lena turning a blind eye to some of those red flags. But she didn't, she'd barely spent 10 minutes with him. So I don't buy the argument she'd be surprised Seymore would be willing to blackmail her. If you want to play Lena as desperate enough to take the chance anyway (or just plain stupid enough not to see the obvious) that's fine, but I reject the idea that this was some masterclass in subtly, allowing the victim to hide in plain sight. It was - by design - the opposite.