I think you’re overestimating the contemporary influence of Jesus. He wasn’t a tremendously notable figure to the Romans at the time - he was, to borrow a term from Monty Python, a wabble wouser. From the Roman point of view, there is some guy with a relatively decent following. Not unimportant, but hardly top headlines of the day, and hardly unprecedented.
The reasoning for them noticing him (as I understand it) was wabble wousing at the temple around Passover and having some followers in town - it wasn’t like Pilate was intimately familiar with Jesus. He was just reacting to a potential riot that some guy named Jesus might have been starting. Around Passover, they certainly wouldn’t want to string him up for a few extra days as a warning to anyone, any more than they would want to keep up a thief.
As for the Romans not knowing anything about medicine… they may have had a primitive grasp on it, and there probably wasn’t what passed for a doctor on scene, but I would go out on a limb and hypothesize that they knew that blood coming out was bad (or good, depending on your perspective). :-p
Someone mentioned something about the wrist damage… well, in a situation where one would be cutting a wrist, you cut along the veins, not across them. If you cut across, it is easier for it to clot, and you have a less likely chance to die. If you cut along them, you open a great deal more tubing to the air, and virtually nothing can help you. I’m not a doctor, but getting a nail shoved through your wrist falls into the former category. Also, with puncture wounds, you have a much greater chance to live if the object remains in the wound (which is why it is funny to see in movies when people just yank stuff out of them), as is the case with the nail - it blocks the bloodflow and lets it clot more easily. Don’t get me wrong, I’m sure it hurts like all hell to have a nail go through your wrist and then support your body weight from it, but you won’t bleed to death in seconds.
As for surviving the entire process - OK, I can see surviving most of the individual parts of the experience, but added together, and throwing in shock (considering that the upright, arms stretched, naked, and in the air would be the absolute worst position to experience shock), dehydration, and other factors, there is very, very little chance. Additionally, if he had simply passed out (which is quite likely), he would have died rather quickly from asphyxiation…
While there is evidence that people have been buried alive, the usual causes for this are diseases or brain damage. Specifically, there are a number of ailments that put you in a coma-like situation - almost no breathing, low blood pressure, all that. In the ancient times and middle ages, when diseases spread like wildfire and people generally didn’t want bodies laying around, they would indeed be buried rather quickly whenever possible. The odds of this are fairly low, though. I’ve never heard of this kind of situation resulting from massive body trauma.
OK, so the process isn’t more thorough than decapitation, but all of it together will get you pretty dead.
Them whacky Romans!