I guess growing up in a harsh world of 'take or be taken from', I can understand why Elsa didn't even consider joining the family peacefully from the start, not even taking the moment to negotiate peacefully about the circumstances, since she never really 'negotiated' in her lifetime. When someone or something is taken from you, it's reasonable to think you need to get it back, but if you don't calm down and think, that thing you're chasing will just get further and further away from you until it's out of your sight for good.
That very thing could have happened with Elsa and Luna. Reda being the too kind for his own good guy that he is to be the one to mend things between them is pretty much expected at this point, but I think Luna suddenly calling Elsa 'Mom' after all that so soon is a bit much.