I am also going to point out that I don't believe there has even been a hint that Halsin is going to be a companion (I honestly wouldn't want him to be either, he's an Archdruid, that sounds like someone far above us in terms of level, he makes sense as a mentor and guide, not someone equal to us) it's just a thing people want. Karlach at least has datamined stuff and a mention in the related MtG set, whatever that's worth.
Agreed. And this also brings up the general issue I have with NPCs like Halsin and even Jaheira being brought in as party companions: how does one justify their being at roughly the same level as the PC? The same even for Minsc, though at least there I can see some hand-waving that can be done to (lamely) justify it.
