The time machine is kind of broken right now, so I'd put up your savefile on the savefile troubleshooting topic so Jan can help you getting out of Amethyst cave.
There's a help center in Kristiline from which you can do quests, I believe. Also, there's a whole lot of side content next to the story itself, so check places you can go to but haven't been able to get to before, etc.
No, you don't need waterfall for either of those. Waterfall isn't an obtainable HM in the game yet. Having the itemfinder to find the kingdra wing is useful though.
Nah, she's not related to the odd key. The odd key can be found somewhere else in Narcissa's gym, and she has nothing to do with it. Curious though, that's the first time i've heard about this girl.
So much to do, so little time! I still need to study some more for my finals, then I need to work on my study intake assignments and then some more sprites as well. I should've started earlier...
It's been a while since I've actually played the game since my laptop broke, but when I had to test the game it was solo for me as well and I'm pretty sure I was playing on normal mode, but yeah it's probably intended to be that way since I don't recall Jan saying anything different :V
Update: Appearantly Melia joins you in the battle if you say she's the strongest.
I think you have to talk to her, or you have to stand in the middle of your rectangle part of the battlefield for it to activate. If that doesn't work, you should put your save in the save help topic so Jan can help you out with that later.
No problem!
But uh, I think that that's a bug. You should check if it's a known bug listed on the main post or if it's been mentioned in the bug reporting topic, otherwise you should probably report it.
Take your time! I have finals to worry about too, but I can do things during the evening for now since I have the rest of the week off still. But hey, all help on the list is welcome :^) More people to help means less workload on only a few people
Shing and I are planning to play through the game a second time to find back all the items and take screenshots of where we found them, so we can list them all. If you can contribute to the list with a screenshot, send them to one of us.