One of the reasons Palestine is NOT a State and is not eligible to BECOME one is that there is no way to tell where Israel ends and the fictional Palestine begins. Therefore, there is no such thing as "parts" of Palestine that Israel could be occupying. You can't occupy something that doesn't, and never did, exist.
The only way for Palestine to come into existence with respect to territory (only one of several required elements) is to negotiate a treaty with Israel. Preferably a treaty of peace.
You lack understanding of international law. The UN does not create, define, nor enforce law. Its a schoolyard of states vying for the best toys in the playground.
And no, international law does not "permit" belligerent acts of force by non-state actors or third-party state-supported actors against the sovereignty, political independence, or territorial integrity of another State. To do so invokes the right of self-defense.