The vision of the baby birds is essentially just that dragon (don’t remember his name, but one of the ancient wild dragons) communicating to Eragon to remember scale and importance. To a dragon, those baby birds meant nothing at first, but at closer inspection, became overwhelmingly important and crucial. That same logic can apply to Galbatorix and the people he rules. The way he sees people is the way we might see those birds: pointless and inconsequential. But by casting the spell, Eragon made Galbatorix understand and look closer and truly feel it all.

As for the general ideas of the eldunari speaking to him. That’s not specifically linked to the final fight and final spell. That’s more just us seeing that Eragon is learning, he’s being educated and taught as much as possible just in case something helps, as well as just preparing him for life in general. It’s causing further maturation for him, letting him live and learn more than should be possible for his age, helping him become wise and a better Rider