The green mockup definitely looks less like R2D2, but you're wrong about it not being R2 in the original shot. It is. And I'm sorry, but no matter how many times anyone tries to tweak that shot, the fact that it's Ben, Luke, C-3PO and R2-D2 is inescapable. It is clearly framed that way, unless you're trying to say that Gilbert Taylor framed three of them and some other random droid that just happens to look exactly like R2-D2?

And the alternate shot is not in the movie. There are simply two R2-D2s on set to shoot the turn.