Heathen,
Your appeal to Mosaic law for anything in the matter of the execution of Jesus is pointless...Jesus was killed by the Romans, not the Jews. The Romans, who borrowed, and perfected, crucifixion from the Phonecians didn't give a rat's hairy but about Mosaic law when it came to executions.
The most logical answer to all of this is that Jesus was executed on a cross, with a cross beam, nailed through the wrists and the ankles after having been scourged...this was the favored way of the Romans to execute criminals...and it served as a warning...thus the traditional cross...visible from a great distance, sent a message that a stake in the ground would not...that death was about.