We don't know what Fredo thinks about the drapes. Do we even know that Fredo knows about the drapes?

Fredo is complaining about many things: "Send FREDO off to do this -- send FREDO off to do that! Get FREDO to take care of some Mickey Mouse night club somewhere! Send FREDO to pick somebody up at the airport!" He's been stepped over by everybody, not just Michael.* If Tom or Neri betrayed Michael, it's a reasonable inference that they did it to become boss, because there's nothing else to tempt them with. Fredo can be bought cheaper.

Everything has to lead to the ending. If Fredo had tried to murder Michael, then why such a melancholy ending? Why is Kay saying, twenty years later, "Tony knows that you killed Fredo," instead of, "Tony knows that Fredo tried to kill me"?

*It's also not hard to scenario that Deanna is Fredo's primary frustration, especially when the deleted scene where they arrive at the party is considered.