It will be Sandbox, but in a way slightly chaptered. Depending in where they wish to go I will treat Locations like chapters, with a big description when they get to cities. But It will be the Players who push the story, with noble acquaintences, and different interactions they do will carve their way through the story.
Again, it depends. You should try getting more feedback on the topic. Is the RP going to be sandbox or with chapters? I'd recommend the second one if you plan on making it a small-scale RP.
The main thing to do though is get as much feedback as possible, to avoid wasting your time.
The concept is quite interesting. I've never been a big fan of Pokemon RPs (aside from the academy and mystery dungeon), but the idea is fresh and I think quite a few RPers who are Pokemon fans will enjoy it. I'm not sure if I can join any more RPs myself, though. Spreading yourself too thin is a recipe for disaster.