"Who is going" versus "who are going"

(Grammar Questions I Couldn't Answer)

This question came out of a grammar exercise from Life Elementary Textbook, p. 97.  In the exercise, the students had to put the verb into the present continuous form.  The context was a telephone conversation.  The question was:

Sorry, who _____ (speaking)?

The correct answer, of course, is "Who is speaking?"

One student was confused, and asked if it shouldn't be "Who are speaking?" because "who" in this sentence is synonymous with the listener, and so should take the second person form, not the third person.
 "Who is speaking?" really means "who are you that is speaking?" and the pronoun "you" of course always takes the auxiliary "are".

No, no, I started to explain, "who" always takes the third person verb.  But in the middle of my explanation, I suddenly realized this wasn't true.

The pronoun "who" can be used with either "is" or "are" or "am" depending on what "who" is referring to, right?
That is, you can say, "Who are they?" if "who" refers to a plural number.
 But you can also say "Who is he?" when who refers to a singular number.
Or "Who am I?"

But then, you can't say: "Who are going out tonight?"  Right?  I mean, that just sounds wrong, doesn't it?  You would have to say, "Who is going out tonight?", even if "who" in this case is referring to multiple people.

Later, I asked this question around the teachers' room.  One of my colleagues from the American South suggested a variant "Who all is going out tonight?" (or  "Who all's going out tonight?" in the contracted form) but this is still the same problem of "is" being used to refer to a plural subject, right?

What's going on here?