I agree somewhat with both POVs here.

Altho it would have been difficult to tell the Bocchicchios story any way other than from a narrative POV- i.e. Vito, or maybe Tom, "telling the tale", which might have bogged things down quite a bit, or flashbacks (which really weren't a feature of the first film) - I think that the reason Michael was able to return from exile was definitely glossed over in the movie and was feature of the plot worthy of an explanation.

For someone who hadn't read the book , it might've caused them to wonder a bit.

Same thing, kind of, with Neri.

No explanation, really, as to who he is or where he came from. He's just there.