Clemenza got very much involved in the killings at the end of GF: He shoots Cuneo or Stracci, he strangles Carlo. In the book he gives Neri the gun to shoot Barzini. So he probably had his part in the planning.
Wasn't it a little dangerous for Michael to have Clemenza involved like that? After all, Tessio and Clemenza were friends who were seperated at a certain time by Vito but after the fishtank episode they could have become allies. Even more after Vitos death.
How could Michael know that Tessio acted alone?
How could Michael believe that Clemenza would be more loyal to him than to Tessio ("for old times sake")?
Even if Clemenza didn't know about Tessio's treachery he could have turned traitor after he got to know what Michael was aout to do.

Clemenza was too stupid to turn traitor

maybe Coppola and Puzo wanted us to think that Michael was smart for making the right choice and trust Clemenza. you don't know how the relation between all those guys went over the years.

First of all, a capo would not be carrying out public murders. Second, who was Michael to trust? Neri was the only person who had personal loyalty to Michael. Third, what was it that Clemenza did that would have made Michael suspicious of him? At some point Michael had to trust somebody.

The novel states that the two capos weren't close. Clemenza lived at the Mall, while Tessio was a near-autonomous operation--so much so that many thought he didn't belong to the Corleones at all. That's the impression Vito wanted to leave. And Vito deliberately kept them apart to prevent exactly the kind of scheming that might have occurred after the fishtank scene.
That scene may have provided sharp-eyed Michael with a hint of things to come. When Vito says to the capos, "Do you trust my judgment?" Clem replies, fervently, "Always, Godfather." Tess just hisses, "Yes-s-s-s." And after the meeting Clem leaves respectfully but Tess leaves in a huff. And of course, Vito warned that "Whoever comes to you with this Barzini meeting, he's the traitor."
I don't think Clem was too dumb to be a Don--I think he was aware of his limitations and content in his loyalty to the Corleones. In the Mafia, that's smart enough to survive.

TB. You'll have to point out to me where in the novel it states that Clemenza lived at the mall. In fact, in the novel right after Sonny learns that Vito was shot, it describres Sonny walking out into the mall and it states the occupants of the houses. While one might describe Clemenza as a "retainer", I don't think that was Puzo's intention.

On the broader subject, I think that both the novel and film almost go out of their way to illustrrate Clemenza's loyalty to the Corleones. Once they got past the Gatto disloyalty, Sonny and Michael relied on Clemenza tremdously. Afterall, it was Clemenza's regime that ended up guarding the mall. It was Clemenza that instructed Michael onthe assassination and it was his regime that planted the gun. After Vito died, there not much info on which to base an assessment of loyalty except that it was Clemenza's protege (Rocco) around whom Mike built a regime. In the end, the only bases for any of Mike's suspicions would be following Vito's advice: whoever comes to you with the meeting is the traitor.

I would add to TB's views that Michael's comment that he figured Tessio would be the traitor because it was the smart move and Tessio was always smarter indicates an awareness that Tessio was making this move on his own and woulf not risk bringing Clemenza into it because first he was trying to save
his own ass, and second he would not risk Clemenza not going along with him, and ratting him out to Michael.

Also as much as Michael had any feeling, I always though he held extra affection for Clemenza because it was Clemenza who tutored him in how to kill Sol and McCluskey, and who was really the first person in the family to tell him how proud everyone was of his military service, not to mention he said "your father too." The cooking lesson probably didn't hurt either. We see this again in GF II when Michael reminisces about the house occupied by Pentangeli, pointing out that he was glad it was first passed from the COrleones to Clemenza (Michael obviously had a hand in this) and then to Frankie.

hmmm... see, Clemenza has always been a favorite character of mine. I think alot of the things mentioned: the cooking lesson, the talking about the pride the family had for mike, the teasing about Kay on the phone, etc. are simple attestments to Clemenza's personality. He is just a straight up likable, good guy; while Sal always seems dark and devious (maybe a forshadowing technique by FFC???). long story short though, all Michael needed to prove Clemenza's innocence (sp?) was his father's word. Vito was so skilled with people, Dale Carnegie would grow jealous. Mike knew that what Pop says is GOSPEL... bottom line.

I can't, because the novel doesn't say so explicitly. I inferred it (perhaps incorrectly) from the description on p. 215 (paperback) of how Vito deliberately kept the capos apart, but kept Clemenza closely under his thumb.

1) Clemenza's people guarded the mall? But he's asking: "Who are all the new faces?"
2) "whoever comes to you with the meeting is the traitor." Well, I think this is a hint to Michael. But even Michael and Vito knew that beside this a lot of unpredictable things can happen - even an alliance between Tessio and Clemenza. Both felt that Michael was weak as a Godfather. And the last words Tessio says: "I always liked him (Michael)."
3) It was Tessio's idea to plant the gun behind the "box-and-chain-thing".
4) Yes, Michael had a positive affection to Clemenza which everybody knew. So this could have become his weak point.

Public murder? What do you mean?
Michael had to trust a lot of people at that point: At least Neri, Rocco and Rocco's co-assassin, Tom, Cicci, the unknown killer of Greene, Clemenza. If one of these people lets out any kind of information the whole plan fails. And of all these people, I thought, Clemenza was the guy who was at that time the most uncalculable person, because he had a lot to lose and a lot to win.

Clemenza was loyal and could be trusted, that is all it boils down to.

It's not clear whose people were guarding the mall. Don't forget, Sonny had previously told Tessio to recruit 50 reliable men for guard duty and it was Tessio who greeted Clemenza and Mike after the hospital thing.

Public murder? How about shotgunning a couple of people as they step out of an elevator and in front of an elevator operator in what is either a residential building or an office building where anyone can emerge at any time from a doorway, coming up the stairs, etc not to mention the echoing sound of both shotgun barrels being discharged.

There's a difference between trusting people and having to utilize them as assets to carry out an illegal activity. To the extent that Mafiosi are imbued with Omerta, Michael could trust Rocco et al. But when it came to real trust, it was only Neri who had an endemic loyalty to Michael borne of Michael's recruiting him personally and rescuing him from an unpleasant situation associated with his police duties. That's an army of one and Michael needed more. So, who remains as the most eligible loyalist? Clemenza.

Doesnt Michael tell Tom (in part2) that he learned from pop that all the men and their loyalties are based on business? Doesnt this go for any affection Michael may have had for Clemenza? I dont think Michael trusted anyone therefore I dont Clemenza would have been a weakness for him.

Well, it all comes back to who do you trust. You have to trust someone sometime. Mike could think "I don't trust anyone." But, operationally, he has to trust someone at some point to carry out his orders. How does he carry out his program for success without doing so? Of al the peole around Michael, there were only three who gave him no reason to distruct them: Tom, Clemenza, and Neri. Could any one of them or any number of them proved him wrong? Yes. But, how would he have carried out his plans without trusting them?