Some thoughts on English dialects:
Northern American English would be similar overall to the dialects of New England, New York, and the Upper Midwest IOTL. Speakers in the Maritimes may have a bit of a distinctive accent, but it will be seen as normal variety of New England English. Anglophones further in Canada will just talk like Americans - no Canadian rising or anything, since the area is being settled by a different population than historically.
Southern American English would combine what IOTL are considered Midland accents (Philadelphia through the Lower Midwest), and the northern band of OTL's Southern accent. How could Virginia ITTL be considered to speak more like Philadelphia than North Carolina? Essentially, I think there will be three reasons. One is merely political. Two is the DSA will pick up some British pronunciations of particular words (similar to how Canadian English, despite being more like American English, picked up some British-isms), while America's "Deep South" picks up Americanisms. Third is once language standardization begins, the idea of a "cultivated southern accent" will come to the fore, which will probably meld the area together more.
There will of course be outliers in the USA. I expect that people in New York, Boston, Philadelphia, and Baltimore will talk fairly distinctly and similarly to OTL - accents are generally set quite early in settlement history, with later immigrant groups doing little to change them besides adding more vocabulary. Newfoundland will continue to have its own odd accent, and of course there are the francophones to consider.