So I went to try the Tiefling/Lae'zel encounter and tried all the options. All of them on a Of the Ancient Paladin.
The [Deception] choice, once the Tieflings are gone you can free Lae'zel, tell her to leave/join, kill her (you need to target her since she's non hostile, it doesn't cause oath breaking) or leave her in the cage (no Oathbreaking). I think the option should be renamed to Persuasion since you can do what you want with Lae'zel including killing her.
Telling the tieflings to lower the cage so you can all kill Lae'zel doesn't cause any oath breaking. Doing the same thing, but switching side once Lae'zel is free and she ask you to help her kill the tieflings does cause oathbreaking.
Attacking the Tieflings straight up [u]via dialogue choice/u] leads to oath breaking. Attacking them (and killing them) before dialogue ever start do not (bug?).
Failing the Deception choice or leaving (and telling Lae'zel it's not your fight once she's free of the cage) doesn't oath break if Lae'zel die (she is hostile). It doesn't oath break if you attack the tieflings and kill them either (despite them being non hostile before you attack them), but you can using none-lethal attacks to knocks them out as long as you get the killing blow instead of Lae'zel. Lae'zel stays hostile, so you have to kill her (I didn't try knocking her out).
When killing he tielfings via the dialogue choice I saw there was a log message. When highlighted it says you can't use your channel oaths and since you have to talk to the Oathbreaker Knight to switch subclass, managing your descent to darkness seems possible. You can also tell the Knight you will regain your "light" and not accept switching to Oath breaking (but I guess that's not in-game yet).
