mdchachi-sama and toritoribe-sama, thank you so much for your very instructive replies to my questions! Your expert knowledge is much appreciated!
I took some time to read the answers given in this thread pretty carefully and to make sure I understand the explanations. Here is my recap of the main points:
1. の can be used as a pronomial adjective denoting possession/origin, and when used in this way, certain nouns, (which can be understood from the context), can be omitted:
A: これはIMCの製品ですか。
B: はい、IMCの製品です。/ はい、IMCのです。
i) Here, の functions as a pronomial adjective indicating the origin of the product, (i.e. from IMC); 製品 "product" is omitted entirely as it is understood from context.
ii) Note that の here is NOT functioning as a pronoun; it is indicating the point of origin of an omitted noun which is to be understood from context.]
2. When の functions as a pronomial adjective as in #1 above, only nouns representing things (and not people) can be omitted from sentences:
ミラさんはIMCの社員ですか。 (Does Mr. Miller work at IMC?)
はい、IMCの社員です。 ("Yes, he does.")
はい、IMCのです。X
i) の here is not a pronoun, but a pronomial adjective indicating the origin of Mr. Miller's employment (i.e. IMC).
ii) The noun 社員, however, represents a person, not a thing, and cannot be omitted from the sentence, unlike 製品 which was omitted in the example of #1 above.
3. When a noun is modified by an adjective or attributive phrase/clause, の can replace that noun as a pronoun:
A: 本を貸してくれませんか。
B: どんな本がいいですか。/どんなのがいいですか。
A: 面白い本がいいです。/面白いのがいいです。
i) の is a pronoun substituting for "books", and is modified by adjective いい
ii) No possession or point of origin is implied in this use.
4. の at a sentence's end with rising tone indicates a question; this usage is derived (ultimately) from the "explanatory の + です" construction
5. the ん of "explanatory の + です" is a convenient contraction used in speech, but is not used as a sentence ending in standard Japanese, in the same way that "he is" would never be replaced by "he's" as a sentence ending in English.
6. As an example of #5, we recall the plain/direct style sentence in the Minna dialogue (quoted in the first thread above):
「おばあちゃんは、いつ会ったの?」.
i) Had this sentence been in formal/polite style, we would have had: 「おばあちゃんは、いつ会ったんですか?」
ii) Dropping the か and だ of the polite style to render into plain style, we would be left with いつ会ったん as the sentence ending, which (outside of dialect), would be rendered いつ会ったの, which is what we do indeed find in the Minna dialogue.
Phew! That seems like a lot to process for what I had thought initially was a fairly insignificant observation in my studies.
Please, if I've misinterpreted the original answers in my restatement here, please do let me know.
I am learning a lot here. I'm so grateful to the good people of JRef who apparently think that there are no stupid questions.
Thanks very much for being so generous with your expert knowledge!