Yep your right, Shogo isn't a true successor to the style because his master did not become Seijuro Hiko so he can't have the title passed on to him.

On another note, I think it's kinda selfish for Kenshin not become Hiko the 14th, because that means the could the style could end with him. You don't even see if Kenji get's to finish his training with Hiko.