i found a chest in the frozen tundra near the diving spot, looks impossible to get to, is that chest one of the the key fragments?
Yup it is!
It is possible to get but needs a certain field ability. That ability also has an upgrade in the dojo which makes that one way easier to get as well.
last thing i'm confused about is the button in the north part of town, says i'm not heavy enough to press i
That switch needs a field ability of one of the new transformations that can be found in version 0.8.