I am so tired of this whole “town line” thing. The explanation was very hand-wavey: “Oh it’s Ingrid’s leftover magic and there’s absolutely nothing we can do”. Didn’t they say that when Ingrid cast the Spell of Shattered Sight, it took all of her magic? (Hence Arendelle melting)
And the only reason Marian was still freezing was for the OutlawQueen drama. There’s no real reason for it to have happened.