I agree with you, there is no evidence that Galante was ever the official boss of the Bonannos. However, just like Vito Genovese when he returned from Italy, I do think that Galante may have been the most powerful guy in the family at times after he got out of prison. He was able to use that power to gain defacto control of the family when Rastelli went to jail. The fact that he was the power on the street and was making guys and hogging the heroin money pissed off Rastelli and also the other families in NYC, who had never recognized him as boss in the first place. Therefore, it made sense that his murder was not just a Bonanno thing but a collaborative effort involving the Gambinos and possibly other families in NYC.

As for Bonanno himself, when it is said that he was forced into retirement, I think that what is meant is that he was forced to retire from leading the Bonannos and having anything to do with NY affairs. The NYC families could care less if he was still able to run what I would call a mini family in Arizona with interests on the West Coast, that had little if any impact on them. If Tieri went to visit him before Galante got hit I think it was to let him know that Galante getting taken out was not an opening for Bonanno to take control of the family and get involved in NY affairs again.