قبل از آنکه

Yes, 'pish az' is the native Persian equivalent. It is slightly more literary but perfectly correct.

Because the action following 'before' hasn't happened yet relative to the first action; it's seen as a possibility or a prospect.

Not at all! It's the standard informal/neutral version used by millions every day.

Yes! 'قبل از آنکه برویم، باید خداحافظی کنیم' is very common.

'Ghabl az' is a preposition (before something), while 'ghablan' is an adverb (previously/beforehand).

In modern Persian writing, a comma is often used if the 'before' clause starts the sentence.

Usually, even for past events, the verb following 'before' is in the subjunctive. 'Before he came, I left' = 'Ghabl az ān-ke biyayad, raftam'.

Constantly. It's a key tool for setting scenes and temporal sequences in verse.

You can use 'ghabl az' + the short infinitive, e.g., 'ghabl az raftan' (before going).

Yes, it's an Arabic loanword, but it's been part of Persian for over a millennium.

In Dari, it's very similar. In Tajik, they might use 'peš az on ki' more frequently using the Cyrillic script.

No, that is incorrect. You need the 'ān' or 'in' as a placeholder.