God’s ways are higher than our ways. God’s thoughts are higher than our thoughts. We will never, this side of heaven, understand the answers to your questions. That gap has to be filled in with faith.
I think there is a fine line that weaves its way between meditation, speculation and presumption. We are called to meditate and reflect on questions like the reasons for Christ's passion, and doing so can only lead us to speculate as to God's reasons and purposes. This I think is healthy as long as we recognize it as such (a product of our reflection), and don't presume to fully know all the answers, because as you, aimhigh, have rightfully noted, these things are fully beyond our understanding.
For what it's worth, my thoughts on this matter are that Christ left his apostles with the mandate to go forth and spread the word. I suspect that Christ came during the Roman Empire because this was the earliest point in human history from which His message would ultimately be spread to the ends of the earth by His followers, and as such, he was subject to a Roman execution.
Part of the horror of crucifixion is the prolonged nature of the suffering it inflicts. In order to avoid suffocation the victim must shift his weight onto the nails in his feet (or probably more usually his ankles) so that the ribcage can expand and allow the lungs to breathe, but in doing so this causes excruciating pain in the feet. This pain can only be mildly relieved by suspending one's weight from the pierced wrists, but in doing so, restricts the ability to breath. Typically the victims would find themselves shifting their weight back and forth from their wrists to their ankles for a protracted period before death overcame them.
It is noted in scripture that Christ's legs were not broken. This was typically done to "hurry things along," as after the legs were broken, the victim could no longer push his weight up by shifting his weight to the feet and suffocation set in soon after.