Couldn’t the earliest preaching in Jerusalem have gone ahead with the tomb still occupied, if nobody checked or the body had decayed beyond recognition?

That theory requires the disciples to announce a bodily resurrection in the same city where the body lay, within weeks, while neither the Sanhedrin nor the Roman authorities thought to open the tomb. The earliest counter-claim on record, in Matthew 28, is that the disciples stole the body, which concedes the tomb was empty and argues about why. Opponents who could have ended the movement by producing a corpse accused the disciples of theft instead. That is hard to explain if the tomb still held Jesus.