Explanatory の/ん in questions with interrogatives: Two conflicting rules

Question

I'm working through this grammar book, which goes into a lot of detail about the grammar of questions and when the explanatory の/ん needs to be included. On pg. 284, the book says that a question must include の/ん if the following is true:

疑問文中に疑問語 (疑問詞) が含まれている (疑問語疑問文の) 場合

With more details further down the page:

なお、疑問語 (疑問詞) を含んでいてもその疑問語 (疑問詞) が述語に含まれているときは名詞文と同じ構造になるので、「のだ」は必要ではありません。

In summary, a question containing an interrogative that isn't the final predicate must include の/ん. Examples: 「田中さんは何を見たのですか。」「田中さんが見たのは何ですか。」(The book also notes that this rule isn't always followed in beginner Japanese language education, even though sentences that don't follow this rule would sound unnatural to native speakers.)

Edit: Just to clarify further, this rule only applies to questions as part of a sentence's main clause. The book notes that relative clauses would not apply. Example: 「私の留守中にだれが来たか教えてください。」

But, on pg. 287, the book lists the following question forms as valid (where ↑ = a rising tone):

だれが来ますか。
だれが来ましたか。
だれが来ます↑。
だれが来ました↑。
だれが来る↑。
だれが来た↑。

These two pages seem to contradict each other. How can the structures on pg. 287 be valid without の/ん, if the rule on pg. 284 is true? The book doesn't clarify this, and I'm not sure what I'm missing.

Answer 1

They indeed seem to contradict each other. However, the rules described in the preceding pages should not be taken too literally. The book itself offers the following caveat in p286.

本来「のだ」が必要とされる文で「のだ」が使われないことがあります。この場合、「のだ」を使っても問題はないので、学習者は「「のだ」が不要になることもある」ということを知っていれば十分です。

As for the examples you copied from p287, the authors probably needed a few examples with an interrogative that end with か (without の) in order to demonstrate だれが｛来る／来た｝か as invalid in contrast to あの映画を｛見るか／見たか｝ with no interrogative.

While these questions are not exactly invalid, they sound somewhat detached from the situation, as if the speaker is interrogating someone for the purpose of obtaining a fact, indifferent to the situation of that person. Depending on the context, they could even sound a little intimidating.

The following equivalents with ん or の sound more natural in most cases.

だれが来るんですか。
だれが来たんですか。
だれが来るの↑。
だれが来たの↑。

The difference is the presence or absence of 前提 and 焦点 as the book puts them. The who part gets focused by の or ん against the background knowledge that someone will, has, or did come and only their identity is unknown. I suppose it is due to the lack of this focus that questions with interrogatives sound unnatural unless they also contain の or ん.

Answer 2

田中さんは何を見た↑。　　　 ⇒"見た" is a plain-form. You don't need "のだ".
田中さんは何を見たのですか。⇒"見た" is a plain-form. To make this a polite form, you need "のだ".
田中さんが見たのは何↑。　　 ⇒This is a noun sentence, and "何" is a plain-form. You don't need "のだ".
田中さんが見たのは何ですか。⇒This is a noun sentence, and "何" is a plain-form. To make this a polite form, you need "です".

In the Plain-form, "のだ" or "です" are not necessary, while in the polite form, "のだ" or "です" are necessary.

だれが来ますか。　⇒Masu-form, Polite-form, No need for "のだ".
だれが来ましたか。⇒Masu-form, Polite-form, No need for "のだ".
だれが来ます↑。　 ⇒Masu-form, Polite-form, No need for "のだ".
だれが来ました↑。 ⇒Masu-form, Polite-form, No need for "のだ".
だれが来る↑。　　 ⇒Root-form, Plain-form, No need for "のだ".
だれが来た↑。　　 ⇒Ta -form, Plain-form, No need for "のだ".

The word "Masu" implies politeness. so you don't need "のだ".