I agree that the turn of events with Seymour in chapter 9 was in line with what could be expected. (Though I had a more vague idea about it — this was a good summary!)
My main point a few comments before yours, was that Lena — who’s happily unaware that she’s the main character of a porn game — didn’t have as obvious reasons to expect it (and nor did she, as far as I can tell).
When she earlier is confronted with the decision of whether she likes working with him or not, it feels like you choose "I don’t think he’s got any bad intentions" rather than "I don’t mind if he’s got bad intentions". But when it turns out his intentions are pretty shady (which should be clear to Lena as well as he presents his deal and the photoshoot plays out), Lena adapts to it a bit too easily, in my view.
The photoshoot with Axel that turned into sex was also not what she had planned, but that was something she knew deep down was likely to happen, and she had plenty of chances to fantasize about it in advance. Making it possible to think about Seymour during one of her masturbation sessions, as an example — or at least fantasize about his power or the submissive position she’s in during their encounters — would’ve made the events in chapter 9 a bit more dynamic and part of a natural progression, I think. But the chapter 9 twist is an important part of his path as well, so maybe it would’ve worked against purpose.
It’s not a big issue, just nitpicking — as we get plenty of chances to do, considering chapter 10 is probably still several months away.