Help with the final crystals!!!

[Dracian]

I've searching for hours...
I'm missing crystals 4 and 7. I've look around the forum, and it seems like I need the other 6 to get the last one.
I'm pretty sure I've found my missing crystal. I can see a treasure chest near the Mabe Village prison in the past. But I can not for the life of me figure out how to reach it!!!
I can also see stairs nearby as well, so it seems I need to go through an underground tunnel to reach this chest...but the entrance is eluding me. Could someone please tell me where it is?

Also, who wants the broom for the trading sidequest? Can't seem to find a taker.

EDIT: Found who wants the broom and finished the trading sidequests. Was hoping that'd bring me closer to this missing crystal, but no luck there. -_-

EDIT 2: Nevermind. Got the last two crystals. Although I am a bit confused as to what triggered the event in Mabe Village that led to the last two. I've been in the village many times searching. Was completing the trading quests what did it?

[Kukironosuke]

Probably o.o you can only do the prison event after you finish the side trading quests