>>39Regardless of the context, all of the antecedents listed are singular. It doesn't matter if they are implied to be plural. The point is that "they" can refer back to a singular antecedent.
If "everyone" is plural, then why does it take a singular verb? "Everyone sings, everyone reads, everyone takes, everyone smokes." Why does it use the adjective "every", which is only used with singular nouns? Since it is treated as a singular, and since it's an indefinite pronoun without a gender, it's logical to use the singular they with it, as well as with similar pronouns, "everybody", "each one", etc.
But no, I have more. Here are some others:
"I, John Shepherd, might conceal any family-matters that I chose, for nobody would think it worth their while to observe me." (Jane Austen, Persuasion)
"Who is in love with her? Who makes you their confidant? (Jane Austen, Emma)
"A person can't help their birth" (William Makepeace Thackeray, Vanity Fair, 1848)
"She kept her head and kicked her shoes off, as everybody ought to do who falls into deep water in their clothes." (C.S. Lewis, Voyage of the Dawn-Treader, 1952)
"I know I like a person when directly I see them!" (Virginia Woolf, The Voyage Out, 1915)