The composite of Fett in that shot is not reversed, as in the negative being reversed as they did in that one ROTJ shot. No, they actually put the shoulder bells on the wrong shoulders on the costume. The RF stalk makes it clear it is not a reversed image. Also, I wouldn't say they 'CG'ed Fett in.. they 'comped' him in. Its a real actor on green screen stage, not a CG character like Jabba. Boba Fett didn't exist in Lucas' mind until after the original Star Wars came out, though it is true that some initial concepts of Vader were of a mercenary type character, which later was resurrected for Fett. Fett does not appear in the original screenplay for Star Wars. He is an afterthought, put in for the 'fanboys.'
And I agree, the best decision about this scene was made in 1977... it was left out. It adds absolutely nothing and just messes everything up now that these characters are better established in other chapters of the story. That scene makes Jabba look like a joke. It completely diminishes Han's encounter with Greedo, and makes Jabba and Fett a whole lot less fearsome since Han basically just blows them off with a sly grin and a pat on the back. Its just silly. It was the first hint to me when SE came out that Lucas was losing it. (well, actually the first hint was the ewoks).