Cicci had nothing to lose, did he? He didn't REALLY turn on Michael. He said that he had never gotten an order from Michael (although why they didn't ask who gave him orders is beyond me). They couldn't really use Cicci's testimony in the case against Michael, AND he made a deal, so it was really a win-win for him.

As for that unasked question, if they had asked Cicci who he received his orders from, couldn't they have gotten Tom Hagen? Just for Tessio's murder alone, they could have charged Tom. Perhaps no corroboration???