Thanks, but 'twasn't I who noticed Questadt in that scene, it was a sharp-eyed poster on this board at least 10 years ago. It set off a long discussion of some of the questions you raised. Some people speculated that Michael didn't notice Questadt at that meeting because, as we see in III, he had diabetes and had impaired vision. I thought that was a big stretch. Finally, another poster referred to an earlier script treatment in which Michael was supposed to have met Questadt in Havana for some purpose not revealed.

That explanation was credible: there were lots of scripts and revisions before FFC settled on the final cut. I'm guessing that Questadt was in that scene because of the earlier script treatment. Somewhere along the production line, FFC chose to abandon the idea of Michael meeting Questadt, but left him in that scene either because he didn't notice him, or because the Havana scenes (which were shot in the Dominican Republic) would be too costly to replicate if Questadt were edited out.

The boathouse confrontation with Fredo (one of the best in the Trilogy, IMO) had enormous drama. Michael was genuinely shocked when Tom told him Pentangeli was alive in the scene before. Another guess: Since Michael already knew Pentangeli had survived, FFC needed more drama for the Fredo confrontation scene, and gave him the line about Questadt belonging to Roth--it put the final nail into the coffin of Michael's relationship with his brother.