That's the Beauty of these films that we are still debating half a century on
So no real surprise there that our theories may morph into must have beens -- what else is left

1. Fredo also said he never met Ola
2. Michael says Fredo didn't know before his boathouse outburst
3. what was he getting in return for a very reasonable inference that Fredo opened the drapes and why
4. If this inference breaks down or it's not the most likely scenario What would they need Fredo for?

We'd also debated Michael could have spared Fredo but regrettably and sadly murdering his own brother seemed a good idea at that time -- and the stark contrast between Fredo's boathouse outburst and Michael's tormented, teary confession

True when scrutinized carefully, the all kinds of scenarios on guesswork about a sequence in the film -- for and against, doesn't entirely make sense. Then again That's the Beauty of these films that we are still debating half a century on