One of the things that I noticed when Regina went to Wonderland and was talking about the Queen of Hearts was that she seemed to be talking about her with an annoyance in her voice – not in a manner I would expect her to be speaking if she were talking about her mother. I think she feared her mother when she was young and then hated her when she was older. I don’t think there was ever a time where she was simply annoyed by her.
To be annoyed with someone I think you have to be either their equivalent in terms in status but have them challenging your position, or they need to be someone who is simply constantly driving you crazy.
My theory depends on Regina having been the one who sent Cora to Wonderland, where she set up camp as QoH. If Regina eventually gained the power to overthrow her mother, strip her of her powers, (they both use the same tricks, so I think Regina stole her initial powers from her mother), and banished her to another land, then I could totally see her attitude having evolved from being fearful of her to merely being annoyed by her.
I think it seems like the Queen of Hearts is Regina’s mother. The actor is different though. I am not sure what that means.
Dunno that this is technically a spoiler because it’s just about a cast member from an already aired ep, and that info is out there for everyone to see, but just in case, I think they just had a “filler actress” for Hat Trick. So many people knew that Barbara Hershey would be playing Regina's mother in a future ep, so having her name in the credits as QoH would have blown the whole massive reveal that they're not ready to make yet. Even with her face covered and voice muffled to hide her identity, her name in the credits would NOT have gone unnoticed. Also, they wouldn't wanna pay a huge amount to Barbara if she's just gonna sit there for a few minutes and not be seen or heard in that ep, so casting a bit part actor as a stand in instead just makes good business sense. That's why, like I said, I thought the elephant appearance was a reference to “the elephant in the room”, which means something obvious that no one brings attention to by addressing it.