Using "quién" and "quiénes" properly

Question

Am I using quién and quiénes properly?

If I think that there is someone here, I would say

¿Quién está aquí?

If I think that there are people here but do not know how many, I would say

¿Quién está aquí?

If I think that there are five people here, I would say

¿Quiénes están aquí?

If I am having a conversation with a friend and is expecting a singular answer, I would say

¿Quién come cebolla?

If I am having a conversation with a friend and is expecting a plural answer, I would say

¿Quiénes comen cebolla?

If I am having a conversation with a friend and do not know if the answer would be singular or plural, I would say

¿Quién come cebolla?

If I am having a conversation with a friend and asked a rhetorical question, I would say

¿Quién come cebolla?

Answer 1

You are not wrong in your examples. However, you are being too cautious. Quién is generic, meaning it's OK whether you think the answer can be singular or plural, unless you already are talking about several people. For example, this is one case when you should use quiénes:

—Esta noche vienen a cenar algunos amigos.
—¿Quiénes?

Even in this case there are people that would use singular ¿quién?. If you're in a group and you're all having dinner, for example, you might ask:

—¿Quién quiere postre?

even though it's quite likely that several in the group will want to have dessert.

Conversely, quiénes is generic if you don't have any reason to suppose that the answer will be singular, for example:

—¿Quiénes vendrán esta noche a cenar?
—Sólo mi amigo Juan.

As before, you cannot use quiénes if you've just established the conversation is about only one person:

—Hay una persona esperándote en la sala.
—¿Quién?

So there are a few occasions when you should choose one or the other, but for the most part quién and quiénes are both OK, with each one providing a hint as to what you think the answer will be, but not changing the basic meaning.