I totally agree. It seems to be the most logical scenario, at any rate. I know Hewitt's account unmistakably points to summer 1986 as the time they began to look at each other with a loving or sexual (whatever it was) interest. As far as Diana, I'm not sure she ever went into those kinds of specific details (certainly not 'on record') but I believe it is logical to assume Hewitt did tell the truth or remember accurately on this point. Diana had already given birth to Harry by the time she was blushing around him and taking the boys to sit in Hewitt's army tank.
Moreover, it fits with everyone of the many accounts of Diana's life, at least the ones I read ages ago: Campbell, Morton, Seward.
About Hewitt though, yes, his position as officer in the Life Guards undoubtedly brought him into royal contacts since the early 80s, but I always had the impression most of his royal contacts came from the polo circuit. He played polo for the Army team in the early-through-mid 80s and was always seeing the Waleses in that capacity. In fact, he once admitted as much, that the first time he saw Diana in person she was still Lady Diana, and it was 1980, although she and he just passingly interacted then.