(2) Detail-oriented may be a standout SJ quality but I'm not so sure that (1) hardworking, and (3) overachieving, are. There's a slight J-ness to those but not necessarily SJ. For example ENTJs are also classic overachievers.
No, you have to put them all together. If you have a person who is known for their hard work and diligence, for believing in and extolling the virtues of hard work and diligence, who was also an over-achiever in the classic sense (i.e., joined a bunch of shit at school, organized events for underprivileged groups, etc.,) AND notoriously detail-oriented, you have an SJ. You can rationalize all of these individual points away all you want, but if you put them all together the portrait they paint is undeniable. Also, I've always understood these "ENTJs are hard working over-achievers" claims to mean, not so much that they're the kid at school who joins every club and heads up every event because they're obsessed with attaining institutional success, but rather that their work, whatever it may be, is probably going to hold first priority in their lives.
They have in common an absurd tendency to type overwhelmingly S characters as Ns. Harry, one of the most S characters of all time, was frequently typed as INFP in that thread (second-most only to ISFP, thank God, and that doesn't even happen in other threads with less attention.) Luckily, I think people started to see the light after Sim made arguments for ISFP, but still, what the hell is that? Even worse was Hermione. INTJ, INFJ, ENFJ. It wasn't until later that she started getting more ISTJ votes, likely because I and a few others were personally arguing ISTJ from the early stages of the thread. Don't even get me started on Snape. All of these characters were so obviously S that I have no other way of explaining the ubiquitous N typings in that thread.
Someone even tried to call Umbridge an ENFJ, and Draco an ENTJ. Sure, if Umbridge is an ENFJ, then Oprah could be one, too. But If that's the case, then ENFJ is the same as ESFJ for all intents and purposes. And Draco was an obvious ESTJ.
Do you actually see strong Sness in characters that are being typed as N here, or do you just suspect something because less than 75% of the characters are being typed as S?
Both. The percentage is less, too, if we narrowed it down to SJs.
And, um, why would I have a problem with typing Ss as Ns if I didn't think that they were, in fact, Ss?
Yes, she is very "Choleric", which is "In Charge", and fits ENFJ, not ESFJ, which is "Get Things Going" or Sanguine. (They get this bad rap of being so controlling and all, but they are really more people-focused than that, while Oprah is ultimately task-focused, on the surface social level. She is, however, people-focused on the "conative" level).
Those things you bolded are not choleric traits. Perhaps you can argue the megalomania is "choleric," but truly, any type can have an inflated ego.