Amy v Isabelle and after - The Blowup

At about the halfway point in Amy and Isabelle we see the blowup between mother and daughter, and its aftermath. This is, apparently, the dramatic climax of the book, a peak of tension in the middle of the narrative, and all after (so far) seems to be an unwinding. We saw the cold-war standoff between A and I in the first chapter, then moved back in time, then forward until we've just about caught up with the starting point. At this point, we learn that in fact Isabelle learned that Amy was sexually involved with her math teacher from her boss, mill manager/owner Avery Clark, who tells her he's seen Amy in the parked car with Mr. Robertson "having his way with her." I found this not too credible, however - possible, I guess. (quibble: would Isabelle have called him anything but Mr. Clark ca 1975?). Anyway, Isabelle rushes home, has a huge blowout with Amy; part of her torment is that she'd almost rather that Amy had submitted unwillingly, but Amy tells her no, she want to have sex with Mr. R. This is both better and worse, for Isabelle. Isabelle drives over the Robertson's apartment, and tells him to get out of town. He tells her (liar) that Amy was sexually experienced. I guess it's convenient for the plot to simply move R offstage, but hard to believe - I'm sure someone like Isabelle, almost anyone, actually, would have gone first to school authorities and then, possibly, to the police. Isabelle returns home and in a fit of fury cuts Amy's hair (that Amy often used as a seductive screen). We then go through several more chapters of the sweltering summer: Amy's h.s. friend Stacy gives birth to a son, Amy spends some time with Stacy's ex, Isabelle desperately tries to "fit in," both at the mill and in town (the church). Her attempts are very sad, pathetic - an incredibly lonely woman. We still know little about her past, but there are hints that her story is much like Amy's (and Stacy's, for that matter).