that adult Shauna's affair is inextricable from her affair with Jeff as a teenager. There's something oddly poetic to me about the juxtaposition and contrast between teenage Shauna--whose life, prior to the crash, is so-to-speak "just beginning," lots of options, accepted to Brown, poised to excel, etc.--and adult Shauna, who feels trapped, stuck in routine, bored, suspicious of her husband (who, it turns out, isn't cheating: "There's no book club!?"), really a perfect stereotype of the fucking desperate housewives schtick. So in both of these scenarios, one variable and one controlled, she has illicit affairs. I would explain the time spent on her adult affair as not just a red herring or metanarrative jab (I like that second reading, btw) but also a psychological profile of someone trying to fuck her way out of an adulthood by reliving a time when she felt like things were still available to her.
That may have been what you meant by trauma, so sorry if I'm not saying anything new. On the point of trauma, it is suggestive that she repeats her adulterous behavior, repetition being a Freudian thing associated with trauma.
But overall, those adult sequences of the affair were the ones I enjoyed the least, so I take your point.
Also, disappointed we didn't get more of the man with no eyes that Taissa sees. Hoping that doesn't just fall away. Freaked me out the first time we see that fucker.