Staff is not different from other collective nouns: It takes a singular verb when it refers to the collection as whole, and a plural verb when it refers to the members of the collection taken individually.
That is for American English; in British English, collective nouns are generally regarded as plural.
See Collective nouns with singular verbs and plural pronouns.
This is a very good question. In general, you can help yourself work through any confusion by trying to express the idea more precisely or in a slightly different way. I'll use each of your examples to explain this a bit further.
EX1: All lizards have a head width, but that isn't what you measured. You measured the different head widths of a group of lizards, or "the head widths of some lizards." I would at least add the "some", but perhaps it would be better to include the actual number.
EX2: You are talking about the growth rate of all birds, not, say, the growth rates of different species of birds.
EX3: Each of two populations has a mean height, therefore there are two mean heights. You are saying that the mean heights of these two populations are different.
EX4: Dogs have noses is correct, but this is a bit inconsistent. For example, dogs have a sixth sense that lets them know when someone is coming, and they have a nose for trouble. People have minds, but they have a combative streak. So, when you are talking about a shared characteristic of all dogs, you use the singular, but when you are talking about something that each dog has, you use the plural.
Best Answer
“Who” can be made plural the usual way, but that is not what is needed here
This is a plural in the sense that it refers to multiple mentions of (or answers to) the question “who?”.
“Who” does not have a plural form like the way that “is” changes to “are”
Generally speaking, the word “who” is a pronoun. It stands in for the mention of a person or people, but has no power to determine in and of itself how many people the speaker is talking about.
“You” does not have a plural form either, but can refer to multiple people
The word “you” in your sentence does not have to change in order to refer to multiple people. To make the reference more clear, you can add “two” or “all” as in “you two” or “you all” (or, regionally, “y'all”).
If you want to ask about the identity of multiple people, convey this with other words