Learn Polish: Oby zatrzymany powiedział nam całą prawdę. - May the detainee tell us the whole truth.

Zatrzymany is a passive participle meaning "detained." In this sentence, it functions as a noun (a substantivized participle) meaning "the detainee." Because it is structurally an adjective, it declines like one. If the detainee were female, it would be zatrzymana, and in the plural, it would be zatrzymani.

Why is the verb powiedział in the past tense if we are wishing for something to happen in the future?

The particle oby (used to express a strong wish, like "may" or "if only") is very frequently paired with a verb in the past tense, even when the wish refers to the future. This is a fixed grammatical pattern in Polish that you simply need to memorize when using oby.

Is zatrzymany the exact same word as "arrested" in a legal context?

No, zatrzymany specifically means "detained" (derived from the verb zatrzymać, meaning to stop or detain). It refers to someone being held temporarily for questioning. The word for an officially arrested person is aresztowany. Notice that both of these words are participles that frequently act as nouns.

Could we put the pronoun nam at the very end of the sentence?

You could, but it would change the intended meaning. Short personal pronouns like nam naturally belong right after the verb or near the start of the clause. Moving nam to the very end of the sentence would put a strong, dramatic emphasis on it, implying that you want him to tell the truth specifically to US, as opposed to someone else.