Recently noticed that Tessio is introduced in the novel like this:

"Then he called the safety-valve caporegime in Brooklyn, a man of unquestioned loyalty to the Don. This man's name was Tessio."

I guess we should have known right there that Tessio would be the one.

True enough Pete. Also, Clemenza was described as immensely tall.

More revealing: The same passage said that, because Tess didn't live at the Mall, some outsiders were unaware that he was under Vito, which Vito wanted because it made his foes underestimate him.

That leads to an ancillary point: the novel several times says Vito had a policy of making people underestimate him. The upside would be that they'd get careless around him. But, the (unstated) downside would be that they'd be emboldened to make moves on him, whereas if they knew his real strength, maybe they wouldn't try.

Although at the Senate hearing Cicci states that he never talked to Michael, he tells Tom and Tessio that the boss said he'd come in a separate car. He then goes on to say that "Well, that's what he said."

Of course, there are a few scenarios by which Michael's statements could have been transmitted to Cicci through someone else, but Cicci's receiving that info directly from Michael is one of them.

Cicci didn't talk to Michael. That plan was fabricated well before the Baptism. Cicci comes up to Tom and Tessio to tell him the change in plans. Tom says he can't go either. Unless Tom can see some kind of eye signal that Cicci makes behind his sunglasses, Sal was the only one in that scene who just heard the news.