I think the excessive physical trauma is necessary to a degree to emphasize the fact that he's lost his healing abilities. This is a character who used to be able to practically shrug off a gunshot or blade to the gut, but now he can't and that's an important element to convey. Did they go overboard in doing so? That depends on the viewer. I felt they went a bit overboard in showing the damage he and the girl inflicted, a lot of that could have been off-camera and I think it would have been just as effective.
I ordered the book yesterday and am looking forward to reading it. We'll see how I feel afterward.
I love the hell out of Touch of Evil, too. But the casting of Heston is definitely curious. Was he that popular an actor at the time that they wanted to cash in on his fame? Were there no Mexican actors who could have played that role?