I said in chat, but what if Regina did not curse Marian but is the reason she was cursed? We know at one point that Regina met Mally and that they were “friends” for several years. That they traded curses.
What if Regina revealed a crush on Robin to Mally, and Mally wanted to help Regina (in her twisted careful what you wish for way, Marian ends up cursed by Mally who is in the original Sleeping beauty tale, a bad fairy. Someone told RH where to find the wand to cure Marian, what if that was Regina? Also, Regina does not have to be the one to kill Tamara. Who still may not be RH’s daughter, and besides Tam went way off the reservation as she would have killed RH too and kidnapped Regina’s son. I don’t see Tam being RH daughter as a ship killer, only if Regina killed /cursed Marian then kills Tamara too.
Another notion, what if Marian’s curse was Robin’s price for his magic bow–nothing to do with Regina, but she helped him to get a solution (though her aim was to annoy Rumpel).
“If I had a world of my own, everything would be nonsense. Nothing would be what it is, because everything would be what it isn't. And contrary wise, what is, it wouldn't be. And what it wouldn't be, it would. You see?” -- Lewis Carroll, Alice's Adventures in Wonderland & Through the Looking-Glass