Does fasting cure severe constipation?

No. If your constipation is severe, go ahead and take a laxative (if it's safe for you to do so). If that doesn't work, you may want to see a doctor or try an enema. If the constipation is not severe, adding fiber to your diet (including a fiber supplement) can make a big difference. A magnesium supplement may also help, as well as probiotic supplements. Also, be sure you're staying well hydrated. Constipation sometimes occurs when a person isn't consuming enough fluids.

No it will make your poo harder. You need a laxative. Drink plenty of water.

Not necessarily; you need laxatives or suppositories.

Depends. Sometimes just being off solid food will get the bowel moving; sometimes you need a stool softener or laxative. But if you don't fix your diet, the problem will only return once you start eating again.