No doubt we can make a case for anything and we still are

I don't think the house would get secured before the party started and then left alone. I reckon someone would have been guarding the Doors all the time and vet people being let in However you are correct just shows how loose the script is respecting these matters.

I reckon huge risk for Rocco or a security person be present around their Boss' bedroom after the Boss' wife had retired for the night and the precision timing.... Kay just turns her back.... for the drapes opener as lucab19 posted even with ninja-like stealth skills

True Fredo only tells Michael useless information but he didn't know that. Questadt was so ineffective anyway but he obviously
1. had prior knowledge of the Senate hearing
2. withheld information that may very well be detrimental to Michael
If he hadn't already taken the necessary measures, bringing Frankie's brother over

While accepting that there are things we don't know and we don't and moving on to the drama between the brothers is fascinating enough but constructing a series of melting pot musts to try to solve the mystery, is definitely more challenging, infinitely interesting and more fun

We could keep going for 2000 years and still no right or wrong answer!
That's the Beauty of these films that we are still debating half a century on

As regards ghost-like, Bussetta, perhaps a possible contender but then he could have killed Michael, single-handed
There were plenty coat hangers and pillows, his preferred choice of killing but then again drama