1) Did AG the show followed closely the book? Oh absolutely not my friend, absolutely not. Season two for example has nearly nothing from the book except character names. Even season 1 is only half the book, the rest was changed or invented. So not at all. I would say that AG the show is a "faithful" adaptation in the sense that the first season at least recreated perfectly the feeling and the tone and the ideas of the story, while presenting fresh characters, new scenes, unseen twists and the like. But it was not a "faithful" adaptation in the sense of book-to-screen-copy. Trust me, reading the novel and watching the show are two VERY different experiences.
2) No he was not a co-writer per se. Neil Gaiman's official role was that he was one of the "executive producer" which in facts meant exactly what he said on the Tumblr ask I linked previously. He had a chat and talks with each season's showrunner, to explain to them his work, to clarify if they had question or doubts, to share his reactions about their ideas and decisions. He gave advices about some stuff he saw sometimes, but overall that was it. He was never a writer of the show, he was never the showrunner of the show (though some media articles hyped him as the "new co-showrunner" for season 3 though in fact he kept an executive producer role, the real showrunner was Charles Eglee), and while Fuller and Green (showrunners of season 1) were keen on having his opinion, we know for example that he was almost entirely absent from season 2 due to working on the "Good Omens" television series at the time (where he was actually a driving power, writer and showrunner, unlike for American Gods).
So yes, Neil Gaiman did gave his creative power to the showrunners. He made it pretty clear in all of his interviews. All he could do was give advice and make suggestions (or a stern talking to - for example when he discovered that Iktomi was not played by a Native American actor, he promised to have a chat with the next showrunner to make sure such an incident wouldn't happen). Another example is the decision to have Easter appear in season 1. Neil Gaiman was asked about it - because it completely messes up the chronology of the novel, that starts somewhere in late autumn/early winter, and where Easter appears later in the novel, after Lakeside (here adapted into season 3 of the show). All Neil Gaiman could answer was: he was just as weirded out as the fan asking, because he did not understand why Fuller and Green decided to move the seasonal order, but they did it and probably had their own plans Neil Gaiman was not aware of.
As if the showrunners screwed him over... He was really faithful and admirative of Fuller and Green's work on season 1, in fact he actually agreed to have his novel adapted because it was them. But then the other showrunners that came along, he did not choose them, and he was kind of forced to work with them. While he never said anything bad about them, you could feel he did not had the same enthusiasm - and he admitted himself tired to have to constantly re-explain his story and world with a new person every time (he explained that in a season 3 interview, when mentioning Eglee was planned to take care of season 3 and 4 and how 4 was supposed to be the last season closing the show).
3) The Orlando Jones thing is really simple. I agree he shouldn't reprise the Anansi role in "Anansi Boys" because this would link it to American Gods (the show) and they are precisely trying to avoid that. The Mr. Nancy from American Gods (the show) was designed for and by Orlando Jones. It is the heavy reimagining of Mr. Nancy from AG (the novel) and it is certainly not the character that appears in Anansi Boys (the novel). They are trying to go for the original Mr. Nancy, which is not a tall, well-dressed and imposing man, but a small old guy not an ounce of anger but a lot of laughter, merriment, humor and jokes.
What they are trying to do with "Anansi Boys" here is to adapt the original novel, not create a sequel to American Gods (the show). That being said I do remember there were big talks of Orlando Jones going into Anansi Boys (the show) back in season 2, when the project was talked about and announced - but given all that happened... well you know, plans seem to change.
(Plus, also Anansi Boys is definitively a British story. The main setting of the story - well one of the main settings - is London. Anansi Boys, the novel, is not much American, it is very in the vibes of British fantasy. To give you an idea, the starting point of the story is London, which is then coupled with Florida, and the story alternates between the two before heading for the Caribbean where there's the final act and conclusion. And that's without counting otherwordly places where legends dwell.)
4) Overall, if you disliked American Gods (the novel) there's a good chance you'll like better Anansi Boys. As I have read the two I can tell you that Anansi Boys is pretty much the opposite of American Gods in terms of story-telling. Lot of absurd and comical situations, a much faster pace, the story is very focused... So yeah, while they share characters and ideas, the two are truly "companions" in that the novels work as an antithesis to each other, one doing what the other misses and the reverse. That's why in general people not satisfied by AG find much pleasant AB.