I consider both "Returns" and "Revenge" as "imaginary stories" as often used in the comics, or "alternate universe" stories as in Star Trek and Star Wars -- stories that are BASED on the established characters and conceivably COULD happen, but did not necessarily happen in fact.

The main problem with both "Returns" and "Revenge" is that they revolve to a significant, in some ways, predominant, extent around a character who was not part of the established canon.

The biggest failure is that they completely dispense with what was acknowledged by Puzo and FFC as the primary unresolved storyline coming out of GF II -- the eroding loyalty between Tom Hagen and Michael Corleone -- and basically just write it off as a throwaway plot device to advance the storyline of Nick Gerace.

The administrators of the Puzo estate should have specified that any novel treatment covering the time period between GF II and GF III, had to deal with eroding loyalty and rising conflicts between Tom Hagen and Michael Corleone as a central part of the storyline.