Subject-Verb Agreement – Should It Be ‘IS’ or ‘ARE’ in This Sentence?

Looking on the Corpus of Contemporary American English for sentences containing are you a [noun] who I find only sentences like the second one you wrote.

Are you a person who's a long time here?

Are you a poet who's a doctor, or are you a doctor who's a poet?

Is that who you are, or are you a Christian who sings?

The reason is the one you said: The noun phrase requires a verb in the third singular person. It is "a doctor who is a poet" not "a doctor who are poet." You could say "doctors who are poets," but that would mean that the subject of the sentence is plural.

Are you parents who are looking for answers on [...]?

You can also not use "who is" / "who are."

Are you a parent looking for answers on [...]?

In this case, you would be using a present participle phrase that, as all the participle phrases, acts as adjective.

It is grammatically incorrect, but most people think it is natural and don't care.

The simple subject is thing, which is singular, so the verb must be singular to agree with it. Notice that in the subordinate clause, scares is singular, agreeing with thing.

The speaker got confused because it seems strange to say that a singular thing "is" a plural like oranges. That's more than one orange, right? Perhaps the speaker could revise the sentence so the subject is "the things that scare me the most". But there are two problems with that. First, the idiomatic phrase is "the thing that scares me the most". If you say "the things that scare me the most", that's weaker, because then you aren't pointing out the #1 scariest thing. Second, the speaker starts saying the sentence before choosing the main verb. By the time the speaker has said "The thing that scares me the most", it's too late to change it. At that point, the speaker sees oranges coming up ahead, and scrambles to choose the verb. "Is" sounds wrong because of oranges; "are" sounds wrong because of thing. Oh no!! There's no time to think this through, so the speaker compromises and chooses a verb to agree with oranges, dimly sensing that something is wrong.

And then life goes on.