No, this Texas actually has a better border. The land between the Nueces and the Rio Grande was at annexation, and essentially always has been, overwhelmingly Mexican. By setting these borders, Texas ensures it will be overwhelmingly *Anglo (what ever that comes to mean) through until the mid 20th century at least.
British spelling certainly. British pronunciation - probably not. Look at OTL's Canada after all, where outside of Newfoundland pronunciation wasn't only closer to the American standard, but many American coastal prestige dialects were actually closer to British English.
I do think you'll see some differences though. Non-rhotic speech, which was IOTL common in the coastal south and the upper class, won't go into decline. The Charleston dialect, which IOTL was already heavily British influenced, will keep going strong (I think Charleston will be the DSA's Boston - not the biggest city, but the oldest, with a strong cultural and educational influence on the nation).
I wouldn't bet on it. Remember that until fairly recently, New England was actually one of the most conservative parts of the U.S. Perhaps swinging to the left was only because the South swung so far to the right economically in the latter half of the 20th century.
This begs the question of whether regional political blocks will form ITTL in the USA. I tend to doubt it but maybe I'm lacking imagination.
On a more general note, it's pretty clear that the DSA has reached close to it's final borders. IIRC, everything but Hispaniola (which could join later) is now part of its territory (this includes the guyanas I think). A war between the USA and DSA looks unlikely, Mexico look to be more stable than IOTL, and will probably form a close relationship with the U.S. I also don't see Britain just assigning land that's far off, like Patagonia or Australia, to the DSA, or it developing a truly independent foreign policy until some time in the early 20th century. Thus what you see will be what you get.