Least evil way to become Unholy Assassin?

I play my 4th playthrough. Most probably my last playthrough for some long time - i want to play other games too. So restarting as Durge is not considered option here.

Im in Act3, my party is ”good” - Astarion (Bard), Karlach (OH Monk), Shadowheart (GOO3/Ranger5/Assassin 4) and me (Vengeance pal). I would like to become UA. But in least evil way since my party is good already.

1. Can only Tav be UA? I would prefer to not break Oath. But i will do it if i need to.

2. I don’t want to sacrifice Jaheira or anyone else from my team.

3. I havent recurited Minsc yet.

4. I already have killed Dolor.

5. Will Karlach (or Jaheira in the camp) leave if i become UA?

6. I don’t mind to kill Valeria.