Your questions are flawed.

Ola asking Fredo to open the drapes is total speculation. Nothing even close to that happens in the film. Anyone could have done it.

The Donship isn't brought up (explicitly) and it isn't really the issue. Fredo tells Michael exactly what the issue is - respect. He knows that nobody respects him and seems to think that helping Ola will get that for him.

Let me throw a question back at you: How could Fredo possibly think Roth can give him the Donship?

He knows that nobody respects him. Even Fredo couldn't have believed that Roth could somehow make Tom, Rocco, Neri, etc. step back and allow Fredo to take over once Michael was out of the way. And, if he wouldn't be Don, what incentive would he have to facilitate a hit?

As I've posted elsewhere, I don't believe he was promised anything in particular. He was so eager for something on his own that he jumped at vague promises from Roth and Ola.

I also agree with Pete that the impact of the ending is really minimized if you look at Fredo as just another enemy, rather than a prodigal brother who unwittingly was involved in a coup attempt and ended up losing his life over it.