So Elsa's emotions seemed to be leading to different temperaments in her powers. The Troll King says something about fear preventing her from controlling it, her ice changes colour during one particularly high strung moment in the film, etc. As for love, she'd just had it brought to the forefront of her mind given Anna's sacrifice. Every other moment where she's failed to grasp her abilities, Elsa was under some form of emotional duress, clouding her mind, and therefore her powers. Even after let it go, she again becomes afraid after learning of her side effects on Arendelle. Parts of these other emotions will have remained during the conclusion, but it was her love for Anna that remained strongest, given that she'd almost just lost her. Love's a largely positive emotion, but it can also hurt, so I guess if Elsa's powers are influenced by emotions, it makes sense for love to provide an equilibrium, and therefore balance and control.
I'd like to think her emotions would continue to play a situational part, rather than her simply having fully gained control, but that's delving into headcannon.
As for Disney ridiculing their legacy, I'd say its simply down to their older films being products of their time, just like Frozen is today. People find it harder to suspend their disbelief for the ol' fairytale tropes these days but Disney like to keep them around anyways. So they simply poke fun at them instead.