Supergirl mentions Argo is back in the finale so I am assuming Alura is too.
As for Earth 2. To me this is a whole new Earth 2 based more on the classic comic version. If they really wanted to be clever they could still use Tom Kavanaugh and Violett Beane on Earth 2. Since they are using a lot of legacy characters on Stargirl they could still have Violett as Jesse Quick only instead of Harry they could have Kavanaugh playing her dad Johnny Quick. They could also use new version of Atom Smasher and Obsidian and all the other legacy JSA characters if they really want too.