I don’t think we have any reason to believe that Regina was in hiding from Rumple…Cora didn’t need to hide her, as she’d gotten out of the contract (and possibly done some further mischief we don’t know about to Rumple–there still feels like some missing history). And let’s see…Cora was living a rather public life as Prince Henry’s wife (and thus we can infer a princess in her own right). Oh, and she marries Regina off, again, rather publicly, to a KING. Rumple was probably quite aware of all this. Keenly aware. Intensely aware. But he was clearly not aware of Zelena until he found her in Regina’s chambers.
I think that the contract gives even more weight to my theory. The show would have a serious issue if they didn’t address the fact that Cora DID NOT escape her contract. What she did was basically what Cinderella did. You heard it. The deal was changed. Cora can’t say “Oh yeah, we mutually agreed on a new contract, but I don’t feel like it anymore. So, I’m not paying.” She can’t get around it, and can’t amend it.
Also, there was barely ever a time at which Regina was any place Cora wasn’t aware of until the day Cora was pushed through the mirror. It was more than hinted to that she almost literally kept Regina on a leash, sometimes to the point of force. Yes, Cora put her daughter on display for her own personal gain, but Regina was never out of the way. So, for all intents, Cora was guarding her.
The day the letter was left for Cora could have been the day that Regina’s mother had let her guard down, and Rumpel finally got the chance to make a plan, and get near to her. I still believe that Regina found the letter in Cora’s stuff, after the “murder”.
I don’t know…I just kinda think it would make more sense than Regina somehow getting to Wonderland before she even knew magic like that, and finding the letter there. Then, they’d have to explain why Regina didn’t question her mother’s belongings being there, or why Cora didn’t make a move while Regina (or someone who knew Regina) was in the vicinity. Even if that wasn’t the case, what people are suggesting about getting the letter to/from Wonderland would take a little time to explain well. Most likely, time that A and E won’t spend because they’re focusing on everything else. I’m just scared of crowding in more plot points, and contrived writing.
The only way I’d see this as being Zelena’s is if Regina somehow got to the letter before it was sent to Wonderland. Even so, I enjoy more the idea that Zelena was so desperate that she’d even mistake her sister’s praise for her own.