Here's my tuppence worth Qckrun:

1. One of the mistaken perceptions that people pick up from the GF film is that when a Don died or retired he was automatically replaced by his eldest son. Puzo points out in the novel that this was not necessarily the case. It was usually whichever member of the "Family" hierarchy was strongest at the time, sometimes this was a son, more often not. Puzo and FFC saw GF as the story of a king with three sons, hence the family succession thing.

2. Sonny was dead, Fredo was weak and stupid and Tom wasn't Sicilian. As FFC said (and I parapahrase here) "One son had his (Vito's) passion and aggression (Sonny) one had his sensitive side (Fredo) and one had his cunning and ruthlessness (Michael)".

3. Haven't a clue.

4. Puzo freely admitted that he wrote the GF to make money and pay off debts (rumoured to be gambling debts).