"Where does Marcus sleep?"
Translation:Ubi Marcus dormit?
Yes, you are right. Latin's logic is that the thing you want to emphasis comes first. Word order is more like a writing style. But they do have some popular style or order (but still not a rule). For example, after question words like ubi or quid (or etc.), you are more likely to see the verb placed after them like "Ubi dormit Marcus", or, adjectives are more likely to attach to the nous they modify to avoid confusion. But again, it's still no rule, just a popular style.
Marce is only used when it's the vocative, meaning you address directly to Marcus.
Marcus is used for the nominative, the "normal" case when you say the name, or when it's the subject of the sentence.
Hello, Marcus! -> Vocative, you talk to him. 2nd person singular.
Marcus is sleeping -> Nominative (=subject) you talk about him. 3rd person singular.
Why is "Marcus" in the nominative case and not dative/ablative?
Like-- I understand that dative/ablative would usually be translated with "to/for/by" or something, but I thought that Marcus was the indirect object? I.e. if "where" is the subject, then how can "Marcus" also be nominative?