Learn English – the correct form to address the “teacher”

There's nothing grammatically wrong with what you said, and there's nothing stylistically wrong. However, if I were to hear someone say "I have neither a brother nor a sister", I'd expect something like "but I do have a {dog/cat/hamster/gerbil}" next.

I don't know where your English teacher is from, but most of the American- and British-speakers I know would never say anything so formal as ""I have no siblings". Siblings is too formal and scientific-technical a word for most of us. Most native speakers would probably say "I don't have any brothers {and / or[CHOOSE ONE]} sisters".

Everyone has their own idea of what is proper to say and what the proper way to say it in any particular context. There are no stringent rules in English about these types of idiomatic expressions. I, for example, would probably say "I have no brothers or sisters", rather than "I don't have any...". I might, if a little tipsy, say something like "I'm brotherless and sisterless, but not dogless", but that would be a weak attempt at humor.

I'd suggest that "I don't have any brothers or sisters" is the norm for most native speakers, and I recommend that that's the expression to use, but I'm sure that others will chime in here if it isn't for them.

"Educated at university" isn't a special expression; at university is simply a spatial modifier showing where the education happened.

It's the same as saying something like "made in China" or "bought at the mall".

(a) [does it mean they] necessarily graduated from/received a degree at the named university,

It implies it, unless further information has been given to suggest they didn't. He was educated at Oxford is taken to mean He has a degree from Oxford University. I suspect the assumption is that if you didn't complete your education or didn't pass your final exams, you don't count as being "educated" there.

Note what I said about "unless further information has been given to suggest they didn't". Context is everything, and you could of course say something like, He was educated at Oxford for the best part of a year before the stress got to him and he returned to driving taxis.

Like with a lot of other constructions, you can also deliberately mislead people: He was educated at Oxford could mean that he didn't get his degree, or that he got it from Oxford Brookes University (rather than The University of Oxford, which is the big prestigious one), or even just that he went to secondary school there.

The deception arises from the fact that you would be using the literal definition and the fact that the literal definition is not how the phrase is normally understood.

(b) can it only imply the person graduated from the named university,

(c) can it mean that the person studied there and left (either to another uniiversity or simply dropped out), without having graduated,

Answered these in part (a).

(d) can it mean a current "undergraduate" or "graduate" student (or their BrE equivalents) at said university?

It would help if you provided a full sentence as your example, rather than just a phrase. If you say he was educated at UniversityName, you mean that his education there has finished.

You could theoretically say he is being educated at UniversityName to suggest an undergraduate. However, I'm not sure I've ever seen this phrase used in this way; it would tend to suggest something along the lines of corrective education (because of the passive tense - compare it to he is learning Subject at UniversityName) and so you'd be more likely to see it referring to primary or secondary education.

Postgraduate study tends to be even more self-taught so in you would be even less likely to see the passive phrase being used in that way. In my opinion saying "he was educated" to refer to postgrad study would be extremely unusual if not outright wrong.