So what's Herobrine's part in this? Clearly he's somehow in league with the Piglins but how? And why?
He's the one who kickstarts the plot, so more likely than not, he's the one behind everything, including the Piglin Army. Maybe he's the true leader of the army? With the Piglin Emperor as his right-hand man. Assuming he's possessing the top half of the Pig Boss, this would make this theory more likely. As for why he's in league with them, that question is just as enigmatic as Herobrine himself.
Looking back at Find the Pieces, it shows that one of the three thrones in the Nether is empty and has cyan wool similar to Herobrine's shirt and hood, which confirms that he was partially in charge of the army alongside the Piglin Emperor and the Wither Skeleton King. As for why he's involved, that's anyone's guess.
Possibly answered by "Rising Kingdom", which shows the King finding the staff he later used to tame the Ender Dragon; it's strongly implied it belonged to Herobrine.
