Questions about The Godfather Part II

Who hired the people who machine-gunned Mike’s bedroom? Roth or Pentangeli?

Did Michael Corleone really tell the Rosato Brothers to “say hello” to Frank Pentangeli? Or did Roth tell them to say that?  Why?  Why would it matter if Pentangeli was going to be dead 30 seconds later?  Or was the entire plan to almost kill Pentangeli, leaving him to think that Michael Corleone turned against him, making Pentangeli open to turning Michael over to the Feds? 

Why did a contingent of police march into the hospital just as Roth was being smothered?