Sebelum mesyuarat, saya menulis semula laporan itu.

Sebelum means “before.” It introduces a time-clause that something happened prior to another event. Here, sebelum mesyuarat = “before the meeting.” You form it as sebelum + noun/noun phrase.
Example: Sebelum tidur, saya membaca buku. (“Before sleeping, I read a book.”)

The comma separates the fronted adverbial (time) phrase from the main clause, clarifying that sebelum mesyuarat modifies the whole clause. In informal Malay it’s optional, but in writing it’s recommended whenever you start a sentence with a time-phrase to avoid ambiguity.
Without comma: Sebelum mesyuarat saya menulis… might read as “My meeting…,” so the comma helps signal the pause.

Malay verbs do not change form for tense. Instead you rely on time-markers (like sebelum, sudah, telah) or context. In our sentence, sebelum already tells you the action happened earlier, so no extra tense marker is needed. You could add sudah or telah for emphasis:
Sebelum mesyuarat, saya sudah menulis semula laporan itu.

Yes. Kembali also means “again.” Menulis kembali laporan itu is grammatically correct and conveys the same idea.
Subtle nuance: semula is slightly more formal/pure Malay, whereas kembali has Sanskrit roots but is common in formal registers too.

Saya is the subject pronoun “I.” Malay allows dropping the pronoun if context is clear, but in written or formal contexts you usually include it.
Omitting it: Sebelum mesyuarat menulis semula laporan itu. sounds like an instruction (“Before the meeting, rewrite the report”) rather than a statement about yourself.