The way I see it is that Fredo was so consumed with being stepped over, courtesy of Johnny Ola bumped into me in Beverly Hills -- Roth knew that is the only carrot to get him, Roth's pawn, on board

As we debated in "Something in it for me" thread: Roth's empty promise carrot was Donship

All Roth wanted was Michael dead exactly what he set out to do and his job done
He goes back to living his retired investor on a pension exactly as he planned

If he got any Corleone business or in the unlikely event that Fredo became Don, his coglioni would be forever in Roth's pocket.
Don Fredo [albeit Roth's puppet] It's never been done before. Honey Bunny for the great man, Mr. Roth.

I reckon, we see enough of the Mafia that anyone born and bred, would know they can't set up shop, outside their Family, on their own including perhaps as one of Roth's "friends in Nevada"? or Havana even with Superman!

It is also too distant Fredo wants respect! now and within the Corleone empire

However if Fredo wasn't expecting the Donship, interested in your explanation as to
1. Why did Fredo think Johnny Ola wanted him to open the drapes in Michael and Kay's bedroom?
2. why bring up the Donship?
3. Why would the Donship be even an issue?

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!