A body does stiffen. It's called rigor Morris. But this is only a temporary effect. And you are right, the raising from the grave is complete nonsense. If someone dies that is effected, the turning rate in most books is 3-6 hours, but since there have (obviously) been no actual zombie reports, we don't know this is for sure. And normally a person gets buried a few days after they die. Plus most present day coffins are concrete. And the ones that are wood are a very dense oak wood. Almost impossible to break through. And then climb through six feet of compacted dirt? I doubt it. The arm thing COULD potentially be possible because of the way the virus works. It might be easier for it to just stiffen all the muscles at once and then be selective in which ones to move and operate, which is why the arms would stick out.
As for saying you could shoot it on the lung to kill it, I dont think you even read what I posted because I already explained this. it is an animated dead body. It doesn't breathe. It is just a bung of electronically controlled muscles.