Even so, how do you know that the empty tomb isn't a fictional element added in later?
Gordon, this is a question that would appear to have been asked for centuries. What is clear is that had it been 'a fictional element added in later' the Jewish authorities would have wanted to make sure that the story was quashed very early in its existence. After all, it was they who had insisted on the death sentence for Jesus, and so they would not have wanted anything that questioned their decisions to become popular.
After all is someone wanted to create a fictional narrative to 'up the ante' with regard to Jesus being divine after he was killed and inconveniently, for them, remained dead, then 'but the body disappeared from the tomb and later on he had a pint with Frank and Neil' is surely to sort of element that would the job nicely, and impress the gullible.
The problem with this is that there had been a number of 'Messiahs' over the previous 50, perhaps even 100 years, many of whom had been executed by the authorities. None of their supporters had suggested that any of them had been resurrected - and in view of the fact that they had all been political/military power-type Messiahs, it would have made more sense for this resurrection story to have ben used for them.
As it stands the 'empty tomb' is just a claim and not a historical fact, and you are failing to recognise this when you frequently challenge people to 'explain' it. Your challenge assumes there was an empty tomb that once contained the body of Jesus - but if this aspect of the story isn't historically true and is fiction, which is a risk, then your challenge to others is worthless.
OK, its a risk, but one with such low probability because of the centuries'-worth of challenges that have been directed against it with no positive outcome for the challengers. As such, it, and all your other challenges end up with remarkably low probabilities, suggesting that the given story has a higher probability of truth than any of the stories proposed by folk like yourself.
You need to get from claims to facts (since we are talking tombs and dead people here) before explanations are demanded of others - but since we are stuck at claims, and bearing in mind what this story seeks to establish (that Jesus was divine, was dead and was resurrected), then the most likely explanation is that some or all of this story is fictional propaganda.
See above for the refutation of your 'most likely explanation' claim.