I almost forgot one more thing. It is widely believed here that Ola's men lied to Fredo about the hit by telling him it would be a kidnapping, not murder, hence Fredo's dialogue during the late night phone call. If all Fredo did was provide Ola with info about the compound, this fits perfectly with the kidnapping hypothesis. Just giving away the layout of the compound in order to snatch Michael would seem harmless to Fredo. Asking him to sneak into his brother's bedroom and open up the drapes would raise a red flag for even someone as simple minded as Fredo. Again, Fredo opening the drapes still is plausible in this scenario, but I think the idea of him simply supplying info about the compound seems more plausible yet.