Depending on time frames, various ethnicities including Semitic and Sea People groups occupied areas of the Levant. The Levant has changed hands, and the people have intermarried with various conquering nations.

Palestinians are a mix of ethnicities from all over the Muslim world and did not exist as an "ethnicity" until relatively recently in history.

So, as you point out the right of discovery has largely been lost. And truthfully, such rights are rarely recognized in history as evidence by the fact that virtually no nation in the world is prepared to give up the land it occupies to previous people groups, they displaced.

So, in reality, the most common recognized right to land is based on the right of conquest. When it comes to "borders"... possession is as they say 9/10ths of the law.

So, to be consistent, let's just acknowledge that the land belongs to Isarel and its citizens as long as they can hold on to it. And any claims by Palestinians not recognized by the government are simply not any more valid, than claims by Germans for the portions of Poland seized by Russia and given to Poland after WW2 or the portions of Poland seized by Russia after WW2.

Borders are constantly changing in the world, and it is silly to presume that somehow the Middle East is different. So, my point is if you want to argue land belongs to "the people that occupied the land first and passed down their claims to their descendants", you are arguing the right of discovery and on that basis, Jews have among the stronger claims, but I don't see the English giving England back to the original people, who lived there or the US returning the land to Native Americans.

A more honest argument is that some nations want to take the land away from Israel and give it to the Palestinians and are simply trying to justify doing so.

Your premise that Palestinians merely changed religion is not exactly accurate. During the Arab Conquest, those people who were not Christian or Jewish were compelled to change their religion, but the Jews for the most part never did and over time were dispossessed of their lands.