I'm not an expert, but I think the influence of Scots and Ulster Scots on American English should also be considered.

And, in New York City and the Hudson Valley, the influence of Dutch.

The author's position on Scots and Dutch is, I believe, summarized by the second (full) paragraph on page 531 (beginning, “Immigration”). Are you saying that on the contrary these languages had an effect on the leveling of transplanted English dialects that went on before the middle of the eighteenth century, or that they warrant special consideration in the subsequent emergence of American regional dialects in the main (as opposed to within segregated communities that code-switched when interacting outside)?

He quotes Franklin saying that in North America you can't pinpoint someone's origin from his speech. But Franklin was born in Boston, where even today, a discerning ear to this day can tell the difference between North Shore, South Shore, Beacon Hill Brahmin and other accents. And there are further variations in Maine and on Martha's Vineyard. I'm no expert in discerning the differences by neighborhood, but I know Boston area people who "pahk" their "cahs", and others who "pack" them (with a kind of long a).

Could be. On the other hand, I've read claims that you can't tell where an Aussie comes from by his accent, but when we lived in Australia we met people with sharp ears for making that very distinction. Maybe Franklin was cloth-eared?

But Martin, arguably a lot of those divergences grew more stark after Franklin's time and are more noticeable now than they would have been in 1770. I would also suspect the massive Irish immigration in the 19th century had some influence on Boston dialects.