You unlock the ? island by getting gold coins. Many of them are hella good so stay tuned for gold coins.
Law is probably one of the best technique characters in the game (after Nami), mostly because he plays almost the exact same way he always has lol. Idk what it is you're struggling with with him, but imo his land combos are better than his air combos.
Btw you can cancel just about every attack in the game with a dash, so if you pull a move you regret, you can just cancel it. They're incredibly liberal with what you can cancel in this game, even many special attacks can be cancelled, which is really nice. There's 0.0% end lag after a dash too so you can do really crazy things with it