Mista Hagen.
Actually there is a scene in the extras DVD, one that I have ALWAYS said should NOT have been edited. A very important scene.
When Michael arrives at the house, after learning that his father had been shot, look closely ( this part is not cut out) when he walks through the door. You will see Theresa Hagen sitting outside the Don's office, and right next to her is Clemenza, who gets up and tells Mike that his father is going to pull through, etc. etc. Now you do NOT see Tessio sitting with them. That alone tells me that Clemenza was being kept out of the loop at that moment. Possibly a suspect by Sonny.
But the confirmation of this is in the edited scene. Michael continues on into the office, and upon entering the office, Sonny and Tessio are sitting there making a list. Sonny tells Mike that he should leave, etc. Mike tells Sonny that Vito is his father too, etc. So smart ass Sonny turns to Mike and says something to the effect of " Ok College boy, who set up the old man? Was it Clemenza?"
And Mike ponders this for a moment and then replies to Sonny that he did not think that it was Clemenza. Sonny confirms this by telling Mike that their man in the Phone company traced calls from the public phone outside the Don's office in the City, and Paulie was the one who made several phone calls and set up Vito. This, in my opinion, is a very key scene that should have never been cut out.
In the book, and Plaw or DonT would know the details much better than I, When Sonny goes alone to the Don's office, he gets the Don's phonebook out of the safe. He calls Tessio and tells Tessio that he wants Tessio's men to guard his father. I believe that he tells Tessio NOT to contact Clemenza.
And even in the movie, it's Tessio's men who are arrested and removed from the Hospital. We never see or hear Clemenza's men being given any orders to be around or protect Vito. Sonny played it safe because as you said, Clemenza was the one who brought Paulie in, and therefore Sonny gave the hit on Paulie to Clemenza.
Don Cardi