Is the discussion of naming Jaime Lannister as Warden of the East in AGOT a retrospective early worldbuilding error or is holding/inheriting a wardenship allowed under Kingsguard vows (Ned seems to think that Jaime will inherit the title Warden of the West eventually as well, making it seem like GRRM hadn’t yet fully fleshed out the details of the situation)?

Probably a mix of each? On the one hand, the Wardenship isn’t lands and castles – it’s a royal appointment – so it doesn’t quite fall under the Kingsguard vows. On the other hand, it’s possible Ned just assumes that Robert will eventually release Jaime from his vows to please Tywin.

But most likely, GRRM just hadn’t made up his mind about that particular aspect of the Kingsguard yet.