Despite the impression the ship tree in game gives that after the BB1 and the BB2 you can pick the other part of the BB ship tree, you can't. There is no switching from the Upper to the Lower BB lines or the other way around.
The in game ship tree is wrong, the Website ship tree is right ( for once)
After the Ammiraglio di Saint Bon you can only choose the Regina Elena and after that only the Conte di Cavour.