1. What you think is a rock spewing green jelly is called "Sensu Beans." In certain maps, map makes input this entity in the map. Once picked up you can use it. This will bring your life back to full.
2. First you gotta be SSJ Goku (around 1.7million ki), charge up the kamehameha wave, and hold it down. Then start pushing the teleport key and you'll be doing it.
3. The differences between what buu changes its' enemy into, if the benefit out of it. One gives recharges ki, one recharges your life, and one adds ki. I'm sorry if I get this wrong but if I remember correctly, Donut (I'm sure of) adds ki on, Cookie recharges ki, and lastly the Chocolate recharges your life.
Hopefully that helped!