"Roth's inside man Fredo's own words" - “something in it for me -- on my own” for Fredo's "little help" to Ola

No doubt the drapes being open is the central event to the shooting up of Michael's bedroom Fredo's "little help"
I too can't see anyone else allowed into their home let alone -- Fredo was the only contender with “the motive and the incentive” for opening the drapes “because he was cooperating with Ola against his brother”

If he had volunteered the information he knew about the Senate hearing which he deliberately and deceitfully concealed even after his betrayal, the Tahoe shooting then "redeeming feature." perhaps could give him the benefit of the doubt but he cunningly withheld it, for Michael to commit perjury and sent to prison, to become the Don-:

Coppola and Puzo did us a huge favour! by stopping - with a strong emotional story, but with the details intentionally left ambiguous. or they didn't have the answers!

The fact remains “He betrayed his brother for personal gain” whether “unwittingly [or otherwise] Fredo was involved in a coup attempt” which could have wiped out the entire Michael Corleone family

He can't admit because of "I swear to God I didn't know it was gonna be a hit,"
can't see him with the shooting itself, and murdering the shooters

As regards concealing the shooters, They're lying there dead! outside his window
I reckon, they may have been told he would help hide them but it was for them to be killed by the third assassin

Who killed the Tahoe assassins-: