I think Fredo was in much deeper than he let on.
Although he says he didn't know it was going to be a hit, then what purpose was opening the drapes going to accomplish?
His excuse is that he was told that Mike was being "tough on the negotiations" and there would be something in it for him,
if he could help make the deal between Mike and Roth.
Given all that,how in the hell does opening the drapes serve that purpose?
Is Fredo that mind-numbingly stupid?
"Hey Fredo,help us speed up the negotiations with Mike. Just leave his bedroom drapes open"
"Sure Johnny,that should work"
Given the years that Fredo brooded over getting stepped over,being relegated to menial duties in the Family,and basically being thought of as a joke,he finally just lost it and
the idea of helping get rid of Mike is not so far-fetched.
I don't think that he would have had any more of a problem getting Mike killed,than Mike had about killing him.
And yes,Fredo was a marshmallow,but helping someone else do the work is a whole nother story.
It's a shame that neither brother could achieve their goals without killing the other one.