The sentence using "who" correctly is:
He is the young man who, you recall, is a 19-year-old who was involved in the motorcycle accident.
In this sentence, "who" is used correctly as the subject of the clause.
The other sentences have issues:
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"Who is Jonathan talking to?" - While commonly used in conversation, it is technically considered informal; "to whom is Jonathan talking?" is the grammatically correct form.
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"Melanie is the lady whom told me to have a seat." - This is incorrect as it should use "who" (subject) instead of "whom".
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"The question is to be given to who?" - This also should use "whom" since it's the object of the preposition "to".
So, the only correct usage is in the third example.