I'm confused by the use of "motive and incentive." Anyone who was assisting Roth would have motive and incentive to open the drapes. We don't know who that might be, but it seems unlikely that Fredo was the only traitor involved in the hit.

If you mean "opportunity," anyone involved in Corleone security might have access to the drapes, including Rocco who is often listed as a possible traitor in the family.

What is Fredo's failure, anyway? He tells Michael he never met Roth or Ola, then lets slip that he's been with Ola in Havana. What else?