I think Love is perfect for Anna, because it can apply to her in multiple ways. In the begining, she didn't understand love, and she rushed off with Hans after just meeting him. In the end, she realized what love really meant, both romantically with Kristoff and how love works in familys' with Elsa. I don't think it works for Elsa because not only did it nullify her powers, she discovered how love was an antidote so to speak, and not the deeper underlying meanings of love that Anna discovered.
For Elsa, yes, I think that freedom could work as her center, but it isn't really what she represents. Freedom is somthing that elsa gained, but it is not what she really shows or teaches the audience. I think the best center for Elsa would be SELF- sacrifice. Sacrifice by itself can mean many things, some being bad, but self-sacrifice is putting others need before your own. If this is her center, it captures all the things about her that people stated above, like being ready to live in solitude so that she wouldn't hurt people, while not mistakeing her center for something else that could be taken the wrong way. It also sound less weird and life-threatning when stated as self-sacrifice instead of just sacrifice. I understand the people who say self-accptance, but Elsa DID accept herself, she just accpeted herself in the wrong way. She understood that she had the powers, but she only thought and understood the parts that could hurt people instead of all the wonderfull things that could be done with them. I think part of the reason this happened is because of the way she grew up, alone, as protection to others as well as herself.
So, overall, I think that Anna's center sould be Love, and Elsa's center should be self-sacrifice.