As far as I'm aware it's not possible to save any Tieflings (besides Rolan, Cal and Lia) and still get Minthara as companion. Using non lethal damage used to work at release, but Larian "fixed" it, so now it doesn't work on Minthara anymore.
Which is quite annoying as it means if you want Minthara in your party, you lose most of the Tiefling related stories in act 2 and 3 as well as Dammon as a trader.
