That is where the balance comes in. I love worldbuilding and I know its importance, but you've also got to ensure that is doesn't take away from the narrative experience but adds to it. Good storytelling and worldbuilding should coexist, and too much of either can spoil things quickly.
Might I add that in cases of the "oldest Nord sources", I assume you are referring to in-game lore books. If so, those books were most likely written after Ysgramor's landing, meaning they were written for a language that is different from Atmoran. They may have simply called the leaders 'Jarl' in those books because that is the closest word available at the time they were written.