Will a relieving letter, stating "this guy has worked for this many years", is enough to be produced as experience letter?

Yes. Any letter from company even an hr letter is sufficient to be produced as exprience letter.

The experience letter is mandatory. A relieving letter works too in case it has all the details of the organization you worked with and your designation as well as your joining date and relieving date both mentioned on that letter. Along with the relieving letter you shall be needed to provide your

Yes. It will be enough provided your designation, service duration and joining date should be mentioned on it. Few institutes strictly demand experience letter though.