1. You should save all of your refines until the Refine Rebate event.

2. It really depends on you whether to get ninjas or obtain items, I generally prefer to spend on items such as Cave Keys, Training Potions, Threads, and Chimes.

3. Yes, you can save the firework boxes to use in the next event.

4. There's no 'mandatory' unit that you really need to get, just continue to play the game and save enough coupons to get a certain ninja you want during Fuku.