First, your English is fantastic! It truly is! :)
Now, onto your questions: I still haven’t read the pilot one-shot that PH ended up being based on, but I know there was some kind of reference or connection to the title in that (I just can’t remember what it was), but I also really do think that Pandora is meant to be a reference to the myth of Pandora’s Box, which contained all the horrors in the world, but also had hope.
This definitely fits the narrative that we see in Pandora Heart’s when it comes to its characters (no one being truly evil, everyone being human) and when it comes to the events that we see take place (from the tragic to the funny, beautiful to the ugly, and sad to the happy). Oz’s character can be viewed as a Christ Figure and a general “figure of hope” and he’s character even gets compared to the “light” (read “hope”) of the Abyss. As for the “hearts” in the title, I think that is just meant to show how everyone’s heart is like a mini-Pandora’s Box. :)
I’m sure there are a variety of other interpretations as well, but that’s how I’ve viewed the meaning behind the title for a long time.
As for Sharon and Break’s relationship. They are my one of my OTPs, I would like to believe that, if the situation had been different (as in, Break’s wasn’t dying and didn’t have such a time limit on his life) that he might have eventually evolved romantic feelings for Sharon, but as it stands, I think he kept his distance from that ever happening. He viewed her as a daughter and sister (as he told Sheryl), and when he finally realized that she was a “grown woman” who could handle heartache and who he didn’t have to coddle, he started to treat her differently - opening up to her more and trusting her with more “adult” information and such, but due to his impending death, I think he always kept a line between himself and Sharon and never allowed himself to cross it with her (since, to me, some of their scenes oozed with a tension that came off more as romantic rather than familial).
Sharon definitely had a crush on Break, but she was aware of his limited time left, and so I don’t think she ever let herself really pursue or think about her crush on him too much. Either way though, I think they loved each other, since they were both very important people to each other.
I hope, in the guidebook, we can learn whether or not Break held romantic feelings for Shelly or not and if he held onto them until the end. If he did, then the tension I felt was likely nothing more than intended ship tease and/or all due to Sharon’s crush on Break, but if not, then there may have been something more that was simmering under the surface, but which neither of the two allowed to boil over.