When Michael said "Tessio was always smarter" was that Tessio was able to recognize change quicker than Clemenza. Clemenza was a drunk. It was he that allowed that little greedy runt Paulie to call in sick giving the Tattaglias the opening they needed to shoot at V Corleone. Clemenza had a heart, meaning he believed Hitler should have been stopped before 1939 but he was not able to see that people were looking to kill Vito in order to bring in their narcotics (opium/heroin) supplied to numb the pain associated with death. Vito sent Luca to the Tattaglias because Luca was dispensable. If you read the novel Luca Brazi was a stone cold sociopathic killer who was suicidal and didn't want his bad gene's to be passed down to a child which is why he had the "prostitute" killed for having his baby.
Clemenza was exonerated for Paulie's misdeeds. And he does tell Sonny that he had asked Fredo whether he should switch bodyguards, and Fredo told him not to. Why do you think he's a drunk?
As for Luca, he was hardly "dispensable." In the novel his unique skills are made even more clear. The negatives you cite made him all the more suited for his role. Even in the movie, Sonny mentions that the Family would be in a lot of trouble if Luca sells out.
