"Whom" or "who" and replies to such questions

Which is the most natural way to ask the question below? Are the replies correct? (Words in parentheses show that they are optional.)

Whose are these notebooks? - (Of) our students./These notebooks are of our students./These notebooks belong to our students./These are the notebooks of our students.

Whom do these notebooks belong to? - (To) our students.

To whom do these notebooks belong? - (To) our students.

Who do the notebooks belong to? - (To) our students.

As far as I know, to whom is a very formal expression. What about the rest?

What about the following question? Is to optional in the three replies?

To whom are you talking? - (To) our students.

Whom are you talking to? - (To) our students.

Who are you talking to? - (To) our students.

There are a lot of questions here, but to answer the first one, the most "natural" way to say this (to my ear) is:

Whose notebooks are these? / They are our students' notebooks.

Out of your choices, the last two questions are the most natural, the 3rd one being more correct but the last one being the more commonly heard.

The construction of our students, without something preceding it, is so unidiomatic as to be wrong. They are the books of our students is right thugh cumbersome; *they are of our students should be avoided.

?Whom are you talking to? is not used, largely because anyone formal enough to use whom will usually avoid ending a sentence with a preposition. (This is a highly controversial point, and I really don't want to restart the argument; I am merely making a point about common usage. My own view, for what it's worth, is that the practice is certainly not always wrong, but is confusing and ugly enough to make it worth advising learners to avoid it.)

And the to in your last answers is both grammatical and idiomatic, though optional. As a point of logic, it might be taken as *I am talking to to our students, but not in the real world.