I mean you have to remember that the people who pay attention to data mining are a small portion of the potential fanbase, and even then, we don't actually KNOW anything about Karlach. We thought we knew about Helia but as you say, she seems to have been replaced (and I'm glad, because a werewolf in the group really would have been too much) so yeah, until Larian announces anything, we don't actually know anything. Assuming otherwise is unwise.
While we don’t know much about Karlach, we can deduce that she is more on the good alignment spectrum since Sven said that the companions that are in the game are the evil and neutral ones, and that the good ones were being saved for later.
Anyway, point being, I don’t think there will be any more companions aside from the 8 we know of. I’d love to be wrong, but Minsc and Jaheira felt very much to me like something they were saving to reveal last.
