Hi, gave it another shot and got a lot further. I found that 6th gem I was confused about in the first area - it was behind a bush. I'd move it out from there, it's tough to find. I got to the area where you have to go down and backtracked like the game said, but I accidentally used up all my kunai and couldn't remove the box to get the big gold gem. I really liked the store though! Getting upgrades like that was fun.
As for a hook, honestly I'd just remove the gem requirement. I think collecting currency is something players will want to do naturally and that having to get a certain number makes it less fun and more like a chore. I think the platforming is enough to drive the player through that section until they get the kunai and unlock the shop. Natural curiosity and being told to follow youkai will hopefully be enough to drive the player.
A few UI recommendations: the Restart and Continue buttons look almost identical right now. I would make the restart button red and add a big X or some other icon that indicates that it's not the option players should pick normally. Or maybe stack them vertically and put continue on top. For the shop: I'd limit the number of fonts and colors going on. I'd say for a project of this length 2 fonts, maybe 3 different styles using those fonts. That way you can have headings, subheadings, and paragraph text.