I'm all for Eva being pragmatic and keeping things as simple as possible within what's reasonable, but I think you're oversimplifying a bit here. Not everyone needs a scene, that's true. And it's easy to either not include Robert or give him a scene that doesn't impact any of the other paths. (That, however, depends on whether Eva keeps to what seems to be the plan, that Robert is out of the picture if Lena and Ian are in a relationship. And I think it's generally better if some of the sex scenes are mutually exclusive, which makes our choices more important and impactful. But yes, he can be kept out of the episode, or he can have a sex scene that doesn't significantly affect the rest of story.)
But what about Seymour? Eva
can limit his interaction with Lena to a phone call and some interior monologue on her part afterwards. I don't think anyone is arguing that every character needs a scene in each chapter, but still it would just delay things until chapter 11. And in that case Eva will probably also have to tip-toe her way around the issue when writing chapter 10, because of how important it is to the story—which could eventually lead to more work than if she just starts dealing with it now.
The thing is that Lena's deal with Seymour doesn't just affect the relationship between the two of them. It also affects Lena's relationship to all the other photographers, and her job at the café, the restaurant and on Stalkfap. And Seymour's threaths also involve Ian and his career at the magazine and the writing contest. And if Lena rejects the deal, it will probably open up some other work related options that won't otherwise be available. (How could she otherwise afford a tramp stamp, nipple piercings and more horse sized dildos?
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Also it will probably affect the relationship between Lena and Ian. Does she tell him about the deal with Seymour? And if so, how does Ian react? Will it differ depending on whether the relationship is open or exclusive? And if Lena doesn't tell him about it, will he eventually find out? Or will Seymour even tell him about it, to show his dominance and clear away one of the obstacles that stands between him and Lena?
It's not like all of these issues have to be handled in chapter 10, but eventually I think they will, and to me that sounds extremely complex, and something that demands some serious planning on how it will all play out, in which order, and how one thing affects the other. Looking at the variations of a particular scene is useful, but it's not even close to illustrating the overall complexity of putting everything together.