Rocket7 Careful, there are problems with both of those first two points. First, while it's true that "who" refers to a subject rather than an object, this rule applies to the position the word appears in, not the word it refers back to. For instance, we say "Who were the investors?" since "who" is directly serving as the subject. But if the sentence is "The investors are flying in for a tour" and we add a modifier, such as "one of whom lives in Brazil," we don't necessarily have to use "who," because the word is not serving as the subject in the sentence. What "one of whom" means is that of the group of people, one has the trait we're about to describe. It is not serving as the subject for a verb, so "who" is not appropriate. We would also say "One of them is visiting," not "One of they is visiting." Again, "they" is the correct subject pronoun, but the subject here is "one," not "they/them."
Second, "who" is not singular. It can be either singular or plural, depending on the intended meaning. We say both "Who is the leader?" and "Who are the winners?"