As has been noted in this and other threads, the evil entity in the whole mess was Leno’s agent, Helen Kuschnik. According to the book, after Kuschnik extracted the deal with NBC to have Leno replace Carson, she planted a news story that NBC executives wanted Carson out. Only days later, Carson surprised everyone (including the network) by announcing his retirement. The author’s implication was that no one from NBC had made anything more than idle chatter about when Carson might leave, and certainly no one had tried to do anything to speed up the process.
It seems clear to the author that Leno himself was never involved in any of the backstabbing, but also that he had a complete blind spot to what Kuschnik was doing on his behalf.
Carson never expressed a preference for either Leno or Letterman, and apparently no one from NBC bothered to ask his opinion.
Kuschnik became executive producer of the Leno version of the Tonight show. Again according to the book, Leno had wanted to say something nice about Carson on his opening night, but Kuschnik vetoed it on the grounds that there would be a clean break between the two shows (and besides, Johnny hadn’t said anything about Leno on HIS final show). In fairness, the staff was somewhat divided over whether Leno should have paid tribute.
Kuschnik’s antics as executive producer eventually became so bad that NBC fired her, locked her out of the building and told Leno to agree with the decision or else. Leno took the news with surprising calm.