1) She sent Emma down there with Charming’s sword in 122. She knew that Emma survived, and got the egg, so she probably surmised what happened. She had magic after 122 and may have gone to check on Mal after, to make sure things were in place. It’s not like locks exactly keep her out of places.
2) She also set up the magic that “preserves her in whatever form she’s in” to protect the failsafe. So she knows Mal’s still a threat to anyone down there.
3) She would have found another way. But Hook made things convenient. She was probably still working it out–maybe trying to figure out how to trick one of the Charmings into helping her or something. Who knows.
4) Hook’s a useful tool to her, nothing more. However, his knowing stuff about what happened could be dangerous to her (Gold & the Charmings have beaten her often enough for her to know they can’t be let in on what’s happening). So he’s useful as a distraction, and then he’s taken care of as a problem if he dies. Neat solution for her.
5) Who knows…maybe we’ll find out. Hook escaped and GOAT helped him get back up–that’s what matters for now.
No such thing as overthinking it.
Keeper of Belle's Gold magic, sand dollar, cloaks, purple FTL outfit, spell scroll, library key, copy of Romeo and Juliet, and cry-muffling pillow, Rumple's doll, overcoat, and strength, and The Timeline. My spreadsheet: https://drive.google.com/file/d/0B6r8CySCCWd9R0RUNm4xR3RhMEU/view?usp=sharing