I'll go on record and say that Roth/Ola told Fredo to open the drapes so the assassins could get a clear look at what they were shooting. That they simply wanted to scare the hell outta Mike and not actually kill or injure him. Fredo believed them
Obviously Fredo did not kill the assassins but he maybe could have provided cover and an escape for those who did.
Your theory is logically plausible (although it seems like a pretty high-risk way to scare someone you don't want to kill or injure) but I don't think it makes sense in the context of the film.
I could list several reasons why, but before I do can you elaborate on why you believe they didn't really want to kill Michael, and what they felt they would have gained by scaring him? Maybe I'm misunderstanding your argument.
"A man in my position cannot afford to be made to look ridiculous!"
