Frankie should have had more faith in his Don irrespective.....

Even though chain of events including -
Frankie getting little or no help / support from Michael in his troubles with the Rosato brothers all because of Michael and Roth's Father / Son 'alliance'!
Clemenza promising the Rosato brothers three territories in the Bronx after he died was in dispute too

Michael turns up at Frankie's house and asks him to "settle these troubles with the Rosato brothers"
"If the Rosatos killed, or tried to kill, Frankie, it was further proof that Roth was behind the Tahoe shooting"

However I believe Michael was not expecting Frankie to come to any harm other than the humiliating back down, perhaps handing over the three territories and in the process looking weak etc.

In my opinion if Frankie is killed and Rosato brothers, Roth's ally take over the Corleone operations then Michael loses his muscle and becomes just another casino operator, easy pickings for anyone which would be bigger worry for Michael?

As you have said [or similar!] many times, Turnbull, when blinded by revenge and greed it can affect the judgements of even a master manipulator like Michael

I too believe Fredo was going to be spared "but that violent, resentful outburst in the boathouse--Fredo never making an apology, and admitting that he betrayed Michael for personal gain--sealed his fate" Fredo's situation became untenable

My take, for what it is worth!

"Michael was frantic to get Fredo on that plane with him out of Havana" For Fredo's safe passage out
Michael's "get word" to Fredo was to assure Fredo everything is all right

Fredo's true treacherous betrayal only came to light at his boathouse outburst and sealed his fate Fredo's situation became untenable