I'll start off by saying that I don't really know if Chen staged this or not. It's not like I've got proof one way or the other in my back pocket. So anything I say from here on is speculation...
Chen Shuibian is/was the kind of person who would pull something like this. Of that I have no doubt. The big problem with a staged shooting is that he'd need accomplices, and that makes it hard. If just one accomplice blows the whistle, the whole plan is foiled and indeed will backfire.
I already said I don't think Annette Lu is a lying scum-bucket like Ah-Bien, so it's a little hard for me to believe she was in on it. Technically, I think it would have been possible to remote trigger a device under the dashboard to shoot her with some light projectile to make that minor wound she suffered. And that would explain the 90-degree turn that a bullet fired from the street would have had to make. But again, there would have to be an accomplice - Ah-Bien himself couldn't have planted the device, so if it was done, it would require some technically competent person with access who would keep his mouth shut. And again, that begins to sound a bit far-fetched - something from Mission Impossible.
As for Ah-Bien's stomach wound, the easiest conspiratorial method would be for the surgeon to have made it at the hospital. No need for a bullet. There was a lot of suspicion at the time why Chen wasn't taken to nearby Chenggong University Hospital (which is the best in Tainan City) but rather to another hospital 7km away and owned by a good friend of his. Of course, you've got to find a surgeon willing to go along with this, so there's another accomplice.
To me, the most suspicious things about the whole incident was how amazingly good the shooter was (if there was a shooter). Anyone here ever fired a pistol? At two targets in a moving vehicle? With the pistol hidden in your pocket or under a newspaper, rather than holding it at eye level? And managed to hit both targets, all with nobody noticing? Just amazing. I have trouble hitting those stationery cardboard cutouts from 30 feet (less than 10 meters) even when I have time to aim and use both hands. And to top it off, the alleged weapon was some homemade crap zip gun rather than a professional pistol like a Glock or Beretta. That shooter must have been like Antonio Banderas in Desperado.
The other suspicious thing is, like already mentioned, Ah-Bien walking from the jeep into the emergency room. I've never been shot, but I've been in some motorcycle accidents and brought to the emergency room - you just don't hop out of the car and walk after something like that. You let them put you on the wheeled stretcher and bring you inside. Even if the wounds later turn out to be relatively benign, you just don't know that when you've arrived at the hospital - you're hurt, bleeding, in shock, you wait and let the staff take care of you. Of course, if you knew in advance that your wounds (if any) were only superficial, you can walk and flash a V-for-victory sign.
My conclusion - I still don't know. Either it was staged by Ah-Bien and a circle of co-conspirators who have all managed to keep their mouths shut, or we have one very lucky shooter and Ah-Bien was so obsessed with electoral fever that he wasn't fazed even by a bullet wound. Make of that what you will.