I love how you changed history to fit your agenda but Rey Mysterio was involved in one of the top, if not the top, feud in the Smackdown! brand with Eddie Guerrero in 2005. His feud with Eddie took a bulk of Smackdown's TV time which started at Wrestlemania then culminated in a Ladder Match at Summer Slam.
What else was he doing in 2005 before Eddie passed away? He had TV Matches with Randy Orton, was in the Final Four at the 2005 Royal Rumble (along side other "nobodies" like Edge, John Cena, and Batista), and was already one of the focal points during the RAW/Smackdown! feud in 2005 (I also believe he was already added in the Smackdown! team before Eddie's death).
So no Rey Mysterio was not in the "lower mid" card feuding Chavao Guerreo. He was feuding with a former WWE Champion that lasted for 5 months. And also worked other high profile matches on TV and angles.
All signs pointed that Rey Mysterio was bound to become a Champion someday. Perhaps Eddie's death accelerated that but the feuds and matches he was working on already indicated that he was one of the top guys in the WWE at the time.
But to answer the original thread starter's question:
No - because Rey Mysterio was already being pushed at the time
Maybe Rey Mysterio would been factored in the MITB Match or US Title who knows. But keep in mind that Batista did get injury probably also could have affected this original plan. Even if Eddie was alive and let's say he's locked up to face Shawn Michaels at WM, with Batista's injury who would be the next top face of Smackdown! to face Orton? That would be Rey Mysterio and it's not out of the question to think Wrestlemania 22 would have gone this way regardless if Eddie died or not.