The correct answers are Little did they know, their awful garage rehearsals would eventually lead to a platinum record, The drummer, who was to become a global icon, couldn't even keep a steady beat back then, and They were playing their first real gig the following week, so they practiced day and night.

"Would + base verb" is often used in storytelling to talk about the "future from the perspective of the past" (destiny).

"Was/were to + base verb" is a formal way to describe destiny or events that were fated to happen.

The Past Continuous (were playing) can be used to describe fixed plans or arrangements that existed in the past.

Why are the others incorrect?

"Were going to eventually leading" mixes constructions; it should be "lead."

"Would becoming" is incorrect because modal verbs like would must be followed by a base verb (become).