finti, sorry but the way the feet were nailed (position wise) and the fact that the "sedile" was used, does show that it had 2 purposes: yes one for added pain but also one for the support to lengthen the crucifixtion and thus add more pain to the death of the person.
The reason why Jesus was not "stoned" but was crucified was: Jesus was accused of blasemphy by the Sanhedrin(which the punishment for it was stoning) BUT Pilot really wasnt too concerned with blasemphy since he worshiped many gods not the true God. So the Sanhedrin then accused Jesus of treason(him claiming to be the King of the Jews when at the time, Ceaser was the only "king" for the land), Pilot had to take notice of the treason issue should this get back to Rome and Ceaser! The punishment for treason was ....CRUCIFIXTION!