In short: Arab "Palestinians" are mostly grandchildren of Arab immigrants. Period. No, they are descendants of an indigenous population that has been there since ancient times. And even if it were true, how does that make their presence less legitimate than the grandchildren of Jewish immigrants?
Your last paragraph shows your aim. It's about arguing about the land while you ignore the Bible (
Ge. 21:12 "And God said to Abraham, "Be not displeased concerning the lad and concerning your handmaid; whatever Sarah tells you, hearken to her voice, for in Isaac will be called your seed.")
and propagating. Not about the facts.