Learn Turkish: Herkes çok sessiz. Yoksa bize bir sürpriz mi hazırlıyorlar? - Everyone is very quiet. Could it be that they are preparing a surprise for us?

You are right about its standard meaning! However, when yoksa is placed at the beginning of a question like this, it acts as a marker of sudden suspicion, realization, or apprehension. It shifts the tone from a simple question to a speculative one, perfectly matching the English phrase "Could it be that...?" or "Are they by any chance...?".

Why is the question particle mi placed after sürpriz instead of at the end of the sentence like hazırlıyorlar mı?

In Turkish, the question particle mi is placed immediately after the word you want to emphasize. By putting it after sürpriz, the speaker is specifically focusing the suspicion on the surprise itself ("Is it a surprise they are preparing?"). Placing it at the end would put the focus purely on the action of preparing.

The first sentence uses a singular structure for herkes, but the second sentence uses the plural verb hazırlıyorlar. Is this a mistake?

No, this is completely natural. The pronoun herkes (everyone) is grammatically singular in Turkish, so it takes a singular predicate (sessiz). In the second sentence, the speaker has shifted their perspective to the implied subject onlar (they) to refer to the specific group of people doing the preparing, which takes the plural suffix -lar.

Is the spelling sürpriz with two "r"s correct? I often hear it pronounced without the first "r".