Come on, even Al Quaeda as an organization has acknowledged his "martyrdom." Sometimes, you have to go with the preponderance of the evidence. There is no "proof" of the type you like that Jesus or Mohammed existed, but we have the reliable testimony of eyewitnesses, recordings in books, etc. In other words, the evidence that both existed is overwhelming.
In a court of law, a person can be found guilty of murder even without a body. Other evidence is examined, and if adequate, the statement can be made, "A murder was indeed committed."