Translation:Who is the person standing between you?
The answer is in the notes under the light bulb, but they write the question there as:
"Who is the person standing among you all?"
I don't understand why they try to confuse people like this because the answer is the same for"among" you or "between" you. How are we supposed to know the difference? Between and among mean completely different things in English. I think it is pointless from and educational point of view to be left guessing when they have not fully explained that 'between' and 'among' are mutually interchangeable in the Chinese sentence here.
I don't actually know the answers to your questions but as no one has replied, my views on them are as follows:
I don't think that when you place a preposition of location after a noun or pronoun that you need a 的, so I don't think it is implied. I recall in earlier lessons that the character for "inside" came immediately after the noun too. Likewise "below" and "on top" in this lesson.
That does not look right to me because you have the interrogative pronoun 谁 that replaces a noun to form a question, so placing a noun right next to it does not look necessary (or right) to me. Also, the 人 is required as the object of the possessive 的 and has to come right after it.