So far, it looks like, in order to "commit" to companions, we will have to decide during certain moments of Act 1 what we will do with a specific plotline (as there will be antagonistic plotlines); instead of the D:OS 2 way "companions not in your party die during a certain moment".
How is it going to work? Look at the current situation when you side with Goblins during the Grove conflict. - Wyll just leaves the party - Shadowheart is clearly annoyed and gets drunk to forget what did she do - Gale is not feeling that he has done the right thing (and has to be persuaded to stay) So, for 3 of current companions one is already leaving the team permanently and 2 others foreshadow their intention -> "If you keep doing bad things I might leave". Or they simply become more evil...
Now, let's talk about the 3 datamined origin companions and possible 2 non-origin companions.
During the same quest, it would make sense for the Karlach and Minsc to leave as well, considering that they are good aligned companions. I'm unsure about what kind of morality the Helia is going to have. It is not even certain that she will be actually in the game, unlike the Minsc or Karlach. Source: https://www.reddit.com/r/BaldursGate3/comments/jcm2bn/datamining_found_evidence_that_a_datamined/
So, looks like Halsin is going to be in trouble during the later stage of act I... And he will have a tadpole! For me it starts to appear that he might be actually a companion later in the game. That actually would be a very cool thing to happen, since he's really well written and interesting NPC.
I bet that you will have to choose between him and Minthara.
That video shows us that indeed Minthara is coded exactly like a companion. Just like with Halsin it would make sense for her to be a possible companion later in the game (and given the amount of character development both characters have it's very likely)
As for redeeming the evil companions, I do not think that it's a bad thing, if it's written well and if it's done in a logical way. Alignment can change and it's way more likely for someone to change from let's say evil -> neutral, than from evil -> good. Certainly, a reverse process (becoming corrupted) could happen for Gale or Shadowheart...
