As much as I like the whole ‘what if Hans was under brain control’ idea, it entirely rules out the fact that Kristoff was not kidnapped, since the creator said he had no parents, no one went after him, and he never seemed bothered by the idea of being ‘kept’ when the troll first said it.
Also, Hans - to my knowledge - never came into contact with the trolls. They had to be present to heal Anna, therefore they had to be present to brainwash/mind control Hans.
Thirdly… why? I suppose I understand that you’d want to think of Hans as a good person underneath, but this entirely takes away any of his agency as a character - and Kristoff’s, by extension! - while making others ‘evil’.
While I’d love to read fanfic AUs of this idea, I would not like to see it being treated as canon. People can be nasty pieces of work, in different ways, and I’d much rather Hans to be simply someone who plain grew up wrong, but made his own decisions as to how to live his life.
I would also like a sequel where he’s forced to thaw his own more metaphorically frozen heart without it being a romantic plotline (defying the ‘he’s a douche, but the right girl can save him!’ cliche), but that’s slightly beside the point. Other than that he’s still acting on his own agency.