I think the Egon leaving was more that he just picked up all the equipment and took the car without much explanation. Leaving his business partners in the lurch. I think it became an obsession for him amd that may have made his sound nuts.
It does sort of fly in the face of the character we seen in the previous 2 movies, Egon was the level headed scientist and Ray was the more obsessive character, Ray mentions Egon called him up a decade previous and said something about a storm of souls or something along those lines, can't remember exactly what was said. The end of the world signs also lend to Egon losing it and going a bit nuts. Just find it hard to accept that given all the dialogue about multiple Gozer manifestations in the first movie that Egon is suddenly seen as obsessed even after he finds the second entry point. I think with a bit of thought that storyline could have been handled better as to me it makes no sense that the most science based member of the team would suddenly become obsessed and not have anything to back it up with.
The non speaking Egon ghost was not cgi entirely. There was an actor on set playing the role. I don't think he needed to speak, the other characters expressions and emotions were enough to let you know how they all felt. They actually spoke to Harold's family to make sure they were okwith what they wanted to do.
It was likely an actor in a jumpsuit to get visual eyeline and have something for the actors to react to, also the hugging scene would be easier done with an actor there. Probably mainly a CG head replacement, though we'll probably see when the "making of" footage appears online. I think it would have added more to the scene if he had said something, just to hear the voice one last time before departing would have made the scene that bit better.
I think the Shandor scene was meant to be ironic or funny in some way. This guy devoted his life to getting Gozer a way to return and in the end Gozer wasn't interested and could handle everything without help from a puny human.
I just wonder who put him back in the casket he was in back in 1984, he is shown to stir when gozer tries to come through but then goes back to his "sleep" when Gozer gets repelled. Gozer comes through and he's up and about so presumably was also the case in 1984. Every earthquake was Gozer attempting to come through again so he must not have got much sleep. 
Gozer was played by Olivia Wilde and Shandor J.K. Simmons.
