Ways to read Mark Haddon's Weir

Mark Haddon's story Weir, in current New Yorker, is a story in two distinct parts, or movements, and I'm not sure the two work so well in concert though others may disagree on this. The first section, which in fact encompasses about 90 percent of the story, tells of a 50-something man, Ian, walking along a remote region of the Thames with his two dogs, thinking at moments about the difficulties of his life - recently separated from wife of many years and unhappy living alone, deeply concerned about his son who has been out of touch for many years and most likely living in an ashram or in some other unconventional situation and very possible deeply disturbed - and he sees a young woman standing atop the weir that spans the Thames and then sees her lean and fall into the rushing water. Bravely, he races ahead, plunges into rough water and, after a struggle, pulls the woman to shore. All this Heddon narrates beautifully (aside from his abundant use of sentence fragments); Ian gets the barely-conscious woman into his car and, yielding to her faint protests, agrees not to bring her to the hospital; instead, takes her home - takes off her clothes, puts her in dry clothes, blankets, etc. OK, you can only imagine how many things could go wrong here - and they begin to do so. The woman wakes up, it becomes clear that she's mentally disturbed (she rambles on about talking stones and hospitals spying on her), eventually dashes out of the house and away. From this point, it looks as if this will be or could be a story about some incredibly bad decisions: does she perhaps report him to the police? blackmail him? But, no, then we are on to the second part - more of a coda - to the story. Unlike the first part, narrated in detail, we now rush through maybe about 5 or 10 years of life - as Ian and the woman (he's never confident that he even knows her real name) strike up a friendship, get together every couple of weeks for coffee, he tells her about his divorce, about this son's return and then disappearance again, and so on - and story ends with an image of life rushing by us and take us with its currents, obviously like the river than nearly swept them both away. Readers who find the closing segment credible or even likely the closing image moving and beautiful will find this to be a great story; I felt a little bit cut off short, however, as the story so suddenly changed gears or pace, and I thought that Haddon for some reason backed away from the situation or condition that he had so carefully established: as if her were faced with the choice of build this into a novel or wrap it up as a story, and he decided to call it a wrap - still a good story but one that for me did not fulfill its promise.