Process: B-Class may be assigned by any editor. By convention, animation articles should be assessed by an editor who has not been involved with the article; this is to avoid bias and to help keep standards high.
Q. What's wrong with the banner? I added and filled in the B-class checklist but B-class is not appearing on the WikiProject Animation banner on the talk page when I saved.
A. Nothing's wrong with it. The template now only shows the checklist for Start and B-class articles. As this article is a Stub, the checklist isn't displayed. (If you change the class from Stub to Start and "preview", you'll see how it works.)
B1 - is suitably referenced and cited
Q. How much reference is enough - for eg, what about articles using only the animated series itself or online interviews? A lot of pages only use information from these sources, which, although is accurate, may not be ideal IMO. What is the policy in this regards?
A. Policy is to cite anything that might be challenged but, again, this is B-Class not a FAC so some latitude is permitted. The sources you mention are fine.
B2 - reasonably covers the topic
Q. How comprehensive does the article need to be?
A. You are checking that there are no obvious gaps and that the article will reasonably answer any questions a general reader (not a specialist) might have. For example, a B-class article on a character would typically say what company publishes the character, when it was created and who by, and which animation it appears in. Similarly, an article about a publication should say when it was published and by whom, identify key issues and creators, and mention notable characters and stories which have appeared within.
B3 - has a defined structure, including a lead section
Q. Organization - What is the minimum to pass the article for organization? If we go by the template comment, as long as it has sections, it's ok, irrespective of whether those actually work or if they are not ok.
A. Broadly, yes, though if they're ridiculously irrelevant, consider changing them to something more appropriate yourself on the sofixit principle. B-Class is not a very high bar. WP:ANIMATION/SR contains our exemplars as examples of the kind of structure we are ultimately aiming at, so anything in the ballpark of the respective one is sufficient to pass this.
B4 - is free from major grammatical errors
Q. what counts against grammar? - needs a teeny weeny bit of expansion, since this is, from my experience, one of the harder ones to grade.
A. Sure. I wouldn't worry about minor grammatical stuff, spellings errors and so forth. If it makes sense and is reasonably well written, pass it. ("The Simpsons is an animated television series created by Matt Groening") Fail it only if the article is poorly written: "The animated series was from Cartoon Network, and it lasted only seven years before it was canceled."
Q. Do I pass a two-line stub for grammar if there are no mistakes or do I not as there is not much to judge?
A. Don't bother completing the checklist for something that short - it should remain under the "stub" class until someone expands it enough to remove the stub tags.
B5 - contains appropriate supporting graphics, infoboxes, or images
Q. If the page has good images, but lacks a much needed infobox, do we pass it or not.
A. Pass it. Please note that infoboxes not compulsory (as there are articles, like some lists which don't qualify, if it is a more usual article like a character, creator or title then just drop in an infobox yourself - it is the work of moments and there is no reason an article should fail this for lack of an infobox). The fail really only applies if the article has no graphic (infobox, photos, graphics) at all. What we don't want is pages that are a wall of text, with nothing to break it up or add visual interest.
Q. Does a longer article require more supporting materials than a shorter one in order to pass Criterion 5? E.g. is one infobox at the top sufficient for a 12-screen-long article, or does it need something to break up the rest of the text as well?
A. Just an infobox is not sufficient for a longer article. The rest of the text should be broken up a bit as well.