I hadn't read the book, and never wondered at all.

After Sonny's murder, the purpose of calling the meeting of the Families was to end the war, make the peace and get Michael home safely. Vito makes that final point quite clear. Once he & Barzini embrace and are applauded by their 'paisan', we know that Michael's coming home, will cleared of all charges regarding 'this Solozzo business', and will not be harmed. The average movie-watcher who hasn't yet read the book doesn't need the how/why/where of Michael's return, they need to see the story get going.
Having the cut from that scene to Michael approaching Kay over a year later was a brilliant way to accomplish that.

The average movie watcher who has read the book already knows how it's accomplished, and good for them. But it wasn't crucial to the film.

Speaking as the average movie-watcher who hadn't read the book by the first 20-30 times I saw the movie, I have to say it never occurred to me to think..."gosh, how the heck DID they manage to get Michael home????"

When I did read the book and came across the Bocchicchio story, I remember thinking..."Oh...so that's how they got Michael home!!!"

To be honest, I never would've dreamed it had been as complicated as it ended up being in the book. Such an amazing story (or subplot) could not have been glossed over, it would had to have been translated to the screen in great detail in order to be done the justice it deserved.

And there was just no time for it in the film. And the film did not suffer for the lack of it.

Apple