The idea that Palestine (and therefore Palestinians) did not exist before Israel was formed, is just propaganda. If you spent time with any Palestinians (or non Israeli historians) you would know how laughable it is.
There are Palestinian folk songs older than the state of Israel which talk about Palestine. Palestinian embroidery, dancing, cooking, wedding traditions, dialect all existed for hundreds and hundreds of years as specifically Palestinian culture, unique to (and informed by) the land of Palestine, and continue to exist today.
Do you think every Palestinian is lying about this? Has secretly developed a regionally unique and distinct culture, recognised as "Palestinian" by neighbouring Arab countries, in just 70 years?
Sure, as with almost all Arab (and non western) nations, borders had not been specifically drawn to keep people out. And of course politically it was dominated by various empires. But to say this means it didn't exist is like saying Morocco didn't "exist" until it was recorded onto a map in France. Just because Palestine existed previously under the Ottoman Empire does not mean Palestinians do not deserve self determination any more than it means Iraqis don't.
(PS "I do not support Israel's mistreatment of Palestinians. What makes you think I said I did?" - I don't think you support it. I meant "people" in a general sense because there are always people on Mumsnet who find a way to disbelieve the reports of highly credible charities and human rights orgs).