Turnbull is a good man. I believe that story. I believe that Ola wanted him to open the drapes Fredo's little help
I too can't see anyone else allowed into their home let alone -- and No one else had the motive and the incentive

Exactly We can't ignore, his outburst I'm your older brother Mike and I was stepped over! It ain't the way I wanted it!
and he can only get the Donship if the current Don was dead or sent to prison

As we'd debated previously and recently in "Something in it for me" thread: and elsewhere in this one:
Donship was Roth's empty promise carrot

the astute and meticulous Roth would have instructed Ola, to fuel Fredo's resentment and make sure Fredo listened good and inferred [Ref: *Turnbull] the reward is Donship
Fredo was Hooked! otherwise Fredo would be stuck running Mickey Mouse nightclubs somewhere!