Fantasizing about the opposite gender

Q:  

1. I am confused about the types of zina. I am aware that looking at a male, whether it's a picture or physically, is considered as zina. Also, fantasizing about a male is zina. But, is fantasizing about an imaginary male also a zina?

2. When people read romance novels, they get aroused, and eventually, discharge. Is wudhu or ghusl necessary?

Food getting stuck in dental cavities

Q:

1. I have cavities in my tooth, and when I eat, food gets stuck. When I rinse my mouth or use miswak, it's still stuck. But when I brush my teeth, it easily comes out. Should I brush my teeth before I perform wudhu everytime I eat, or is it okay for the food to be stuck in the cavities?

2. Is it permissible to have dental fillings so that food doesn't get stuck, or is the filling regarded the same as stuck food? If it is permissible, is there any specific type which should be used as a filling? For example, the fillings come in gold, porcelain, a composite resin (tooth-colored fillings), and an amalgam (an alloy of mercury, silver, copper, tin and sometimes zinc).

3. Sometimes, the food eaten comes back to mouth unintentionally. When this happens, does wudhu break? What about fast?

Masturbation

Q: After urinating, when rinsing the private area with a hand shower, the pressure of the water is satisfying. Sometimes, I get really aroused, which follows a discharge. Is this counted as masturbation? Do I have to perform ghusl? 

Drinking rainwater

Q: According to a Hadith, rainwater is shifa for all diseases. Does this also apply to drinking rainwater which has been taken out from the ground? Because when it rains, the rainwater passes through rocks and is stored underground and taken out through machines for drinking. So is it necessary to collect and drink the rainwater before it falls to the ground or will the underground water also have the same benefits for Shifa?