Ok, so the idea is that He somehow survived the cross or never went there, then died a natural human death after having a family.
There would then have been no Resurrection and no faith built up around it. Men did rarely survive the cross, but no-one would confuse the slow recovery of a bloody, half-dead man with a resurrection.
The Resurrection was the core of the faith, such that S. Paul said he was on trial because of it. This is not the narrative of a natural survival or of somehow escaping the cross.
If He had gone to the cross, not risen again, and later his wife and child joined Him in the tomb, yes, the Romans would have produced His body, long before the wife and child died.
ICXC NIKA