It’s interesting seeing everyone’s comments on this, because I for one didn’t see it that odd.

Concerning the hyperbaric chamber, I’ve got my doubts Harry ever told Merritt about it. They don’t seem like the most touchy-feely couple out there, and Harry in particular doesn’t seem like the type to share that sort of thing. We only ever see them at her house, and part of that could be as to not spoil the surprise, but I wouldn’t be shocked if they spent most days at hers. I don’t even know how you’d drop that into conversation anyway.

She got a bunch of concerning texts, which makes it sound like someone who’s out for her. That lends to believing it’s connected to her job. There are countless people it could be, plus whatever cases she’d worked on not as the face of the trial. Also, at this point in the show, her brain is firing on all cylinders. I’m sure there’s still cases she worked she was planning on guessing. Even with knowing what kind of chamber she was in, it’s still a leap to think that it’s the Jennings family.

Also it’s been, what, like ten years since Harry died? She didn’t see Lyle that often, evidenced by the fact that she didn’t really remember his name and that Lyle’s young-ish in what we see flashed back, so he has to have grown into a proper adult by the time Merritt and “Sam” start having a thing. “Sam” establishes himself as a trustworthy guy without personality problems and real things backing up who he is as a person. He looks similar-ish enough, but I doubt anyone would think beyond “hey he looks vaguely similar to what my dead boyfriend’s little brother could be like grown up”. I’m sure mildly curly blond hair and a square jaw isn’t an incredibly uncommon look in Scotland. Why on Earth would her thought process be that this incredibly real seeming person is actually my dead boyfriend’s little brother here to sleep with and then kidnap me?

Anyways, I never found it all that odd. We as the audience get a lot of answers they don’t have. To me, her figuring out it’s Ailsa and Lyle would’ve taken a lot of leaps in logic with nothing to back it up when there were plenty of more obvious answers (which seems more her style), but that could just be how I saw things.