No, the additions didn't derail the series. I blame Neil himself, who took over as showrunner for season 2.
I'm not sure where you got this information - Neil has been involved as Executive Producer (which is a title that can mean anything from 'show runner' to 'rights owner' to 'someone who paid for it') but he's not been the showrunner of American Gods at all. I don't think he's had much creative control over it - just vague input at the script stage. He's much more hands on for Good Omens and The Sandman.
Wikipedia says: "[Brian] Fuller and [Michael] Green served as the showrunners for the first season and were replaced by Jesse Alexander for the second season. Charles H. Eglee serves as showrunner for the third season. Gaiman serves as an executive producer. "
Yes, I know Wikipedia is probably not always a reliable source, but them's the breaks. I think I recall reading that the reason they keep changing showrunners is due to studio interference, and Fuller in particular seems to be great at coming up with concepts for shows but never being able to follow them through for very long. (He created Pushing Daisies, Wonderfalls, Star Trek Discovery... and didn't stay with any of them for very long. Some were cancelled after a season or two, some he left during or after the first season. Hannibal is probably his most successful show).
I will agree that the first season of American Gods is the best, and the most faithful to the book - although they did add stuff (mostly side stories and back stories for characters like Laura, Mad Sweeney, and bringing one-sceners like the Djinn and Bilquis in as recurring characters) which seemed to work. Going from S1 to S2 and the lost of the showrunners caused cast members like Gillian Anderson (Media) and Kristin Chenowith (Easter) to decline to return, meaning they had to change Media to New Media, and skip any followup with Easter. This meant further diversions from the book which also kind of meant the story moved more slowly than it should. S3 was supposed to be "back to the book" with the Lakeside arc, but they kept having episodes where Shadow leaves Lakeside to go deal with more god-related things all over the country (whereas in the book, he stays there for the whole winter). There was also a lot more added stuff with Laura, and Technical Boy, and entirely new characters like Demeter, Tyr, and Cordelia which meant moving more and more away from the book.
I think *originally*, back before they'd even started S1, Neil had said that the plan was to split the original novel into three seasons, and then they could continue beyond that with new material following the characters. Obviously this plan changed, as by the end of S3, they're about 2/3 through the book, with the big climax still to come... assuming that the show is picked up by another network, as Starz has cancelled it.
I am somewhat disappointed at where it went - sometimes a book-to-tv show can undergo significant changes and still be a great telling of the story (The Expanse kind of fits here - the diversions from the book add to the characters, rather than divert the plot), but others it just kind of ruins things and slows stuff down, which is kind of what happened to American Gods.
If I was younger Tiff, maybe!!!! it's just down the road.
Now I'm jealous. Very jealous. 
