My own speculation/guess is that the Immobliare story line was greatly expanded when it became apparent that Duvall would not return to play Tom Hagen.

Watching GF II a couple times over the last few days I am more and more convinced that the final half-hour or so of II is specifically setting up a conflict between Hagen and Michael intended to be played out in the next chapter of the saga.

My guess is that Tom Hagen would have pursued alliances with other organized crime figures such as Altobello and Zasa and opposed the entry of Vincent into the family (too reminiscent of Sonny, who never did trust Hagen as a consigliere). The possibility of Tom Hagen joining forces with one of the other families would have been a huge story line.

On the other side you would have Connie pushing for Vincent to enter the family and Michael caught between the warring factions of the family.

All this would have produced enough dramatic material that the whole Immobliare story line would have either been completely unneccesary or at the very least drastically cut down.

(Although it would have made the Vatican tie-in with Andrew Hagen slightly more meaningful if Tom were still alive and active in family operations.)