You raise several good points, especially about Orpheus ha.
I've always headcanoned it that Karlach and Orpheus are uniquely equipped for the job in ways that the others aren't
It's a plotpoint that Karlach is so uniquely tough that she could withstand an unwilling demonic heart transplant(and presumably thrashing around violently the whole time). In scenes regarding transformations the narrator says things to the effect of muscles being replaced with synapses, so is it that large a leap in logic because she's so beefy, Karlach would have better control over herself as an illitid?
Orpheus was born with PLOT MAGIC that makes him immune to elder brain control, so he's a really, really good choice.
Admittedly, both of those ideas have flaws but that's my personal headcanon still.
