Apparently Karlach boarded the nautiloid when it went through Avernus, got captured and implanted while it was there, and hitched a ride out with it. Which honestly sounds like a lie, because that would require a lot happening in a short stretch of time. Like, probably 30 minutes of time in-world. We know she already has a tadpole, but the idea that she got specifically captured and implanted so quickly seems questionable. Unless you subscribe to the theory that true souls are hand picked and they were in Avernus specifically to implanted her.
I agree with you. Its One of the reason I made my post about how the entire events are orchestrated by mephesto AKA Raphael <disguise>. I agree Karach never got on the ship by accident, neither was Gale or Astarion. They are say they are on the ship, but you do not see them, and all find YOU. Astarion talks to you first, gale travels to you, and Karlach threatens to shoot you.
Now we do not know in EA why you are special, but I believe it's no accident that a mythic rank 20 arche devil chooses to seek you out and bring you to the house in their domain. No Cambrian can teleport an entire party to avernus and back without turning into tieflings unless they are an arche devil.
