Namaskar,
Your time period of 250 years is way way way later. We're talking about 2500 years ago, at the time of Buddha.
You disagree on"Yes, we were part of modern day India during the Maurya dynasty"
Exactly....this is where we get f@@ked up.. Nepal has no written history to claim that we were a unified nation then, nor do we have any historical figure to back it up. The western history tells that the Ashoka claimed almost all land in the modern Indian continent, even drove the army of Alexander away fro the Hindu Kush. What are the chances that we not nabbed by Asoka. You give me a proof, and I'll beleive. Many changes occurred after Ashoka's period and his downfall. The fact India should have been a Buddhist county had Ashoka been there for a longer time or his newer generations were strong enough. I think Chanakye turned it into Hindu right after Asoka died. The unified Ashoka nation fell into pieces again into smaller kingdoms. So back to small kingdoms...My assumption is somewhere the line, we slipped off from them and became a distant land. Your time period of 2500-250=2250 years before Prithivi Narayan. That is one hell of long time, anything could have happened. If I'm not wrong, our Lichivi period (traces of it) starts around 1st century. If  that is the earliest history we have, we're still missing some puzzle of 500 years, from the birth of Buddha to Lichivi, did anybody that existed around that time in Nepal fought against the Mauryans? So far I remember, there is no evidence of any Lichivi king who unified or fought with any bigger opponent at that time, we simply have temples and monuments. Burden lies on us to prove that we were always independent. Independent right till our written history starts, what about before that? Simply we did NOT have history does not mean other civilization didn't exist.
Anybody with ancient Nepali history can shed some light on this?
I'm not a history major, simply learning to empower myself, still learning........