The em dash is used correctly in the sentence "The building next to ours—the one with the all-cedar exterior–was engulfed in flames." This sentence uses an em dash to indicate a connection between two independent clauses, where the first clause is "The building next to ours—the one with the all-cedar exterior—" and the second clause is "was engulfed in flames."
The em dash is also used correctly in the sentence "The secret I have to tell you (the one I’ve been hinting about) will surprise you." This sentence uses an em dash to set off a parenthetical element, which is the adverbial clause "the one I’ve been hinting about.", the em dash is used correctly in two out of the four provided sentences.