Car, the other congo
Grimsby, although technically it was not the Third Division South that year. Walsall and Mansfield.
Possibly S****horpe (another chance to show up the word censor) or Lincoln City.
Grimsby Town were original members of the Third Division in 1920-21, but played all their games from 1921-22 to 1957-58 in the Third Division North.
I'm unsure whether that inaugural third division should be considered the third division south or not. There was no corresponding north division until the next season. But I have read that there was a plan to create the two regional divisions all along and that the northern division was delayed because if was more difficult to organise, getting clubs from a number of local leagues, rather than essentially just renaming the Southern League first division. The clubs were all drawn from the south so it was a third division south in that sense, not a national division.
Yes, up to a point you're correct as what remained of the Southern League First Division essentially was absorbed into the Football League as the Third Division for season 1920-21.
But it was never referred to as Division Three South until the next season.
Filbert Street, Highbury, Maine Road, The Dell, Roker Park, Ayresome Park, Highfield Road, Burnden Park
edit: Baseball Ground
All correct, although I've just thought of a tenth one.
Victoria Ground
Thinking through the candidates, its amazing how many of the top club grounds are more than 100 years old, although I doubt many of the orginal stands are still standing. Our East Stand (1934?) must be one of the oldest.