In one of the pictures with Marin I saw there were only 4 instruments to be found, does that mean there are four instruments for each of the characters?
The screenshot only showed 4 instruments because the sub-menu was open, covering up the other four. I don't wanna say too much about that, so you'll just have to wait 'n see! It's nothing as complex as you suggested, but I think my current plan will feel... familiar.
Are the dungeons going to be the same, edited versions or completely original?
So far everything looks amazing. It might be worth seeing if you can lift anything from Oracle of Ages/Seasons also
All dungeons and areas are completely original. There have been some gfx used from the Oracle games, and maybe even stuff (not gfx related) I don't want to reveal just yet.
As Midna pointed out, the torches are from there, as well as the walls. So far, however, most things I've used from the Oracle games have been modified in some way. For example, a lot of the wall designs in the Oracle games contained oddities that you'd never know about unless you studied them and drew them. So things like that have been re-drawn to look better, or to fit within the bounds of how much new space there is.