Seems to me that if the UN has the right to create Israel as a nation it has the right to declare it two.
Nope. The UN did not create Israel, it had nothing more than an advisory role in that process. Britain, which had jurisdiction over the territory at the time, created Israel. Britain got the authority to do that from the Supreme Allied Command, which captured the territory from the Ottomans. The UN had no place in the process other than it proposed a plan that it hoped those with authority would implement.
Now that Israel is sovereign and independent Britain no longer has authority there either--Israel is a sovereign state, master of its own destiny, exercising its own right to self-determination, and so on. More to the point Israel self-declared independence after the British withdrawal, and that declaration was recognized by almost every country in the world--the Arabs refused to recognize it, and invaded. Some of them have now recognized Israel, others still have not, but certainly the UN has no jurisdiction over any of this. In fact, Israel exists even as a member of the UN in any capacity at all only because Israel chooses to.
I think you should look up the word "sovereign" in your dictionary. Unless you want to side with Hamas and assert that Israel does not exist you have to accept that Israel has a sovereign right to decide whether or not it wants to annex any other territories (and those territories have a right to decide whether or not they want to be annexed). Israel will no doubt refuse to, other than on terms of its own choosing (i.e., occupation in perpetuity).
Even if Israel did nominally annex the West Bank / Palestine it could do it under a "one state, two systems" formula, the way that Hong Kong is part of China but not administered under the same laws or the same constitution, with separate currency, separate passports, separate criminal laws and no right of travel between the two. In other words, occupation in perpetuity once again.
None of those outcomes are good for Palestinians. Their best option is to step up and take control of their own destiny, become sovereign in their own right, and manage their own affairs in the way they see fit.
