I don't think Roth feared reprisals because he didn't think anyone would suspect him. As Michael said, "He acts like I'm his son - his successor."
So Roth sends Ola with a positive message to Michael while he's stirring up trouble for Pentangeli in NY hoping to cause a rift between Frankie and Michael.
Sure, there are a few loose ends if you look closely (who killed the assassins) but, overall, it was a brilliant plan which almost worked.
I agree. And, the purpose of Roth's birthday party was to establish an alibi: "What, me kill Michael? Why, I loved him like a son. Just ask those plug-uglies who witnessed me anointing him as my heir at my birthday party."