Learn French: « Quel désastre ! » s'exclame le protagoniste. - "What a disaster!" exclaims the protagonist.

This is a reporting clause, known in French as an incise. When quoting someone's exact words, French requires inverting the subject and the verb in the clause that identifies the speaker. So, instead of le protagoniste s'exclame, we must write s'exclame le protagoniste.

How would this inversion work if the subject was a pronoun like il instead of le protagoniste?

If the subject were a pronoun, you would use a hyphenated inversion after the verb: s'exclame-t-il. Notice that with a full noun subject like le protagoniste, there is no hyphen, and the noun simply follows the entire pronominal verb (s'exclame).

Why isn't it Quel un désastre to match the English "What a disaster"?

In French, the exclamative adjective quel (and its forms quelle, quels, quelles) is followed directly by the noun it modifies. You never use the indefinite article (un or une) after quel, even though English naturally uses "a" or "an".

Are the spaces inside the quote marks and before the exclamation point required?