Board Thread:Spoilers!/@comment-26443838-20151009221621/@comment-23932365-20151010215746

Zelena's time travel spell can be explained. The items she collected to cast it represented love, wisdom, courage, and innocence respectively. Glinda states that they are all symbolise elements of magic, explaining why Zelena was able to break the Laws of Magic and travel back in time; she literally controlled magic itself.

The examples you mentioned, in my opinion at least, are all loopholes to the Laws of Magic. The Curse of the Empty Hearted needs the heart of the person you hate most to create a false illusion of love. In Jafar's case, he broke the Laws and was able to make Anastasia fall in love with him without any sacrifice or price. About the changing the past thing, my guess is that Jafar didn't want to bother actually going back when he could simply alter Alice's memories.

This is all my reasons as to why the Laws of Magic are actual restrictions, because there seems to be no consistent magic system in Once.