Learn Spanish (Latin America): Al final todo salió bien. - In the end everything turned out well.

Yes, but when paired with adverbs like bien (well) or mal (badly), salir takes on the meaning of "to turn out" or "to work out." So salió bien translates perfectly to "turned out well."

Why use the preterite salió here instead of the imperfect or the past perfect?

The preterite is used because the phrase Al final (in the end) frames the outcome as a single, finalized event. Even though your current lesson focuses on the past perfect (había salido - "had turned out") for actions completed before another action, this sentence is just stating the absolute final result of a story in the simple past.

Could I use por fin instead of al final here?

You could, but it changes the nuance. Al final simply means "in the end," objectively stating the ultimate outcome of a situation. Por fin means "at last" or "finally," which usually expresses emotional relief that a long wait or struggle is over.

Why is the verb salió singular when todo ("everything") implies multiple things?

Just like the English word "everything," the Spanish pronoun todo is treated as a single, collective idea. When it stands alone as the subject, it always takes a singular verb conjugation.