I said from the beginning that the QoH is Regina's mother but then I also saw imdb. I know it's possible to lie on imdb, but I don’t think they would go out of their way by hiring another actress just to fool the audience.
Also, Regina’s father, Henry, isn’t a king. Therefore Regina wasn’t royal until she married Snow White’s father, King Leopold. That should also explain why QoH isn’t Regina’s mother–because Regina would have been royal before she got married.
I freaked out as well when I saw the heart vault thingy. My guess is that every fairy tale world has one and whatever is kept inside each box depends on the Queen (or King) of that world. I definitely don’t think it was Regina’s father’s heart inside the box. I think we would have been able to see it when it was on the ground, plus Regina stuck her hand pretty far down so there's open space. Also, in the story of Alice and Wonderland, eating the mushroom (either the left or right side) can make you grow or shrink. So I think she was definitely feeding the mushroom to her father. When he comes to his normal size, his and his daughters reaction would have been much more joyful, in my opinion. They acted like they were still keeping in touch, but it was hard to communicate or something. That may not make sense but it does in my head. 😀
Lastly, timeline-wise, I think this was before Regina kills her father. Obviously because I said I don’t think it was his heart inside, but also because Regina threw her father’s heart in the fire to enact the curse. Sooo I think he’s done-zo.