Pentangeli didn't supply the hitmen. To the contrary: Pentangeli was set up by Roth to be the fall-guy for the shooting. Roth knew that Pentangeli would be at Anthony's party, contentious over Michael's support of the Rosato brothers against him. Therefore, he'd have the "perfect" motivation to attack Michael, and thus divert suspicion from dear, kindly, let's-make-a-deal Roth.

Kay wasn't asleep when the drapes were opened. In fact, she's wide awake when she notices that the drapes are open--how could she notice that if she were asleep? So, the drapes were opened earlier, when the bedroom was unoccupied. Now, there were thousands of guests at Anthony's party. Michael had security--we see uniformed police around, plus Rocco's always scanning the crowd while Michael and Geary are doing their love-fest with the big donation check. We can assume that the house was guarded to keep wanderers and the light-fingered among the guests from strolling in. But that wouldn't stop a member of the family or staff from coming inside and opening the drapes. I think that was Fredo's job.

As Michael told Tom just before leaving Tahoe, "Right now, you're the only one I can completely trust." Logically he couldn't rule out Neri and Rocco because they were his security--and security failed. But his statement, "Fredo? He's got a good heart. But he's weak and stupid--and this is life and death," did not rule out the possibility in Michael's mind that Fredo was somehow complicit in all that happened--that's implied in "weak and stupid," and later in "Tell him I know Roth misled him."

Once Fredo revealed himself as the traitor, Michael had no need to pursue other details. If he thought Rocco was complicit, Rocco would have been dead before he got back from Cuba. Instead, both Rocco and Neri embraced him when he got back. They were both in his good graces.