@ Amy
I think the heart is Katherine’s because there is no real indication that it is any one else’s. I also think it’s Katherine’s because of the parallels between Katherine and Gram. They were both Regina’s pets who decided to ignore her wishes and chose their own paths. Katherine’s purpose in Storybrook was to keep David away from Mary Margaret. Katherine is no longer standing between David and Mary Margaret, so Regina needs to get rid of her. So Regina kills her heart. What better use of the heart then than to frame someone for the gruesome murder?
I agree with this understanding of things. Regina has threatened Sydney before by saying she gets rid of things which are no longer useful to her. Kathryn’s usefulness had simply run out, so Regina decided to make her disappear. That played into Regina’s desire to frame Mary Margaret for Kathryn’s murder.
@RumplesGirl: With regards to what happens to David during his memory lapses, there is an entire thread dedicated to this question: https://oncepodcast.com/forums/viewtopic.php?f=26&t=282
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy