What great finds, Fire-Haired! I listened to each of the links. I love stuff like this.
I agree with much of what you say, but...
I could swear the interviewer in the first link isn't southern, but British born.
I can hear slightly more British Isles-Celtic fringe in the white people from Washington DC and/Virginia in the 1860s, which is really just Virginia, than one can hear in those areas today. That makes sense to me because it was less than 100 years since the split from Britain.
I disagree that people don't sound like that today. I mean, it's not identical, but I think Virginia and Tennessee/Kentucky people people have retained a lot of that accent, although it's become less "British".
Al Gore:
https://www.youtube.com/watch?v=BWN0fdb0jjE
Senator Byrd:
https://www.youtube.com/watch?v=kZI60DP5Oyo
Virginia/Tennesse/Kentucky are not South Carolina or Louisiana: the accent is different. Senator Strom Thurmond is the hypocrite, die hard segregationist who fathered a mixed race daughter. As I often say, you couldn't make a lot of this stuff up.
His speech about not wanting black people using the same pools:
https://www.youtube.com/watch?v=SG-clHmRqo0&list=PLLPBPBGCff2d1gVgXoLF6xzd0c7mI94P7
https://www.youtube.com/watch?v=Ecy0yRpL8cw&index=2&list=PLLPBPBGCff2d1gVgXoLF6xzd0c7mI94P7
His very dignified daughter:
https://www.youtube.com/watch?v=G-hnZVN8lXQ
I still maintain that although everyone speaks more loosely in informal settings, class plays an important part. Someone who has gone to private schools and Ivy League colleges is always going to speak more distinctly and correctly than someone from the working class, no matter the area of the country or the social setting. Southerners are always going to drop more endings and pronounce fewer consonants. I also think American speech, particularly among young people, is increasingly getting the "sound", the dropped letters and slurring of American black speech, partly because they listen to so much "black" music, and partly because black Americans have become so much more visible on the public stage.