So I can confirm that it is totally possible to maintain an Oath of Devotion while siding with the goblins. This should definitely NOT be possible given that one of the tenets of the oath is compassion, which involves taking care of the weak. Destroying the grove involves killing non-combatants, which is the epitome of something that should break this oath, and yet my oath was intact.
To be fair, you do break the oath if you open Sazza's cage, which was likely the devs way of trying to make you an oathbreaker before you go this route. But if a party member opens the cage you are good, even if you agree with Minthara and commit the atrocities. I personally don't think you should be allowed to side with Minthara on attacking the camp, even if you plan to side with the tieflings in the end. An Oath of Devotion paladin should need by oath to side with the grove. Paladins who want to side with Minthara can be oathbreakers or, like her, can be Oath of Vengeance paladins.
