If I remember right I read about this "not Sherlock" months ago somewhere else.
It was said that it's nothing of relevance, as this guy seems to be only a "stuntman", lying on the bed instead of Benedict.
Also Mofftiss explained that the solution is to find while just watching the episode in normal speed, so no stills or slow motions are needed to find out what actually happend.

So this is well observed but probably not the solution. 

It could just be a continuation error. Which I guess they would have to be aware of. Any errors just add to the complexity of the theories once people start to break it down!
Maybe Benedict couldn't film that shot for one reason or another and they used a stunt double.
Here's a production photo of non-Benedict-Sherlock.



So could this be the guy on the stretcher? Just an accidental shot of the stunt double? I can't find a picture of the stretcher shot to compare.
It's Sherlock that gets lifted onto the stretcher how are they meant to swap him out under the watch of everyone but mainly while John is right there? The survival had to happen while John was on the other side of the ambulance building, out of the line of sight.

I think that Sherlock was replaced with a body prepared by Molly after he was taken by those paramedics. (and then that fake dead Sherlock was taken to the hospital instead of the real Sherlock)