While I don't think that deceiving them to save a life goes against the Oath of Devotion (though it is a tricky call with the honesty provision), I agree that having a more clear persuasion option makes sense. The current persuade option kind of implies that you are bluffing to trick them, even though you can actually kill Lae'Zel, which as you said, makes little sense for most characters.
What I think is far more problematic for the Oath of Devotion is that it is totally possible to side with the goblins and murder all the non-combatant tieflings with Minthara while maintaining the Oath of Devotion. One of the key tenets of the oath is compassion, which entails protecting the weak from harm. Siding with the goblins is the greatest possible violation of this.
