Yes the Korok is east from the plateau. He's big so you don't have to pay super close attention to see him if you're in the right area. I never got more than probably 500 Korok seeds in a run and that was plenty to upgrade your inventory a lot. I forget the exact number, but a sizable percentage...
I think that's fair to say. There's a large collection quest which you would discover on the road between Dueling Peaks and Kakariko (or elsewhere if you miss it) as well. Towers, shrines, and divine beasts are probably the best way to track your progress. Most Zelda games you can sum up your...