The whole proving Jesus existed and was crucified seems fruitless.
As I said, evidence points to both as any historian worth their salt would agree to both because both secular and religious historical sources point to both truths. After all, there are very little things you can prove outside a math class.
People are free to disagree, so we will move on.
Assuming the premise that Jesus existed and was killed on a cross, we next have to ask why.
Now Islam states that God took Jesus off the cross so that another could die in his place, but why?
The answer is obvious. Mohammad identified Jesus as a prophet from God, as all religions do around the world, so why would God allow a prophet to die on a cross but not Mohammad?
After all, Mohammad converted with the sword and was a moderately successful warlord. Why? Because God is great and this is how God must work. How then could Jesus be any different? This is why I think the story was changed by Mohammad. Problem is, Jesus was 100% different than this.
Of course, throughout the OT you have other prophets suffering the same fate as Jesus, they were oppressed and martyred. So Mohammad glibly says that the Bible has been corrupted, so you can't take those accounts at face value either, and for the same reason.
But if you are not Muslim, the whole notion that God took Jesus off the cross and placed some other poor sod there is absurd.
But for those who are not Muslim and believe Jesus both existed and was crucified, what reasons other than the Biblical reason is there?