Oh dear. You are literally making my own argument for me and - astonishingly - don't even seem to know it.
...and as such agrees to establish a new national border between the two States, that "could" or "could not" be based onto a former internal provincial border.
Yes, ONE way to draw a new international border is to adopt the former administrative unit. As I have already explained, this is the customary international law of
uti possidetis juris and, as it applies universally. Alternatively, two States can draw a new international border between them though a
treaty agreement which delineates the new boundaries.
As it happens, Israel and Jordan AND Israel and Egypt did both. The international boundaries were established on the MANDATE boundaries (as is explicitly stated in both peace treaties*) and then confirmed on the MANDATE boundaries in the peace treaties. *I posted the text already, you should go back and read it.
A provincial border - whereby the province (Kosovo) is part of a sovereign nation (Serbia) does NOT constitute an internationally recognized national border towards it's legal owner e.g. Serbia. Also not in the case of e.g. Kosovo declaring independence - since it needs the consent aka a treaty with Serbia - whereby the latter might recognize an independent Kosovo
This is entirely irrelevant to the drawing or creation of borders. The legal question of unilateral secession from a parent State, without assent of the parent State is one that has not been solved in international law and there remain a number of dissenting opinions on the matter. Should Serbia accept, or be forced to accept, Kosovo's unilateral secession the international boundaries will be the former regional boundaries, UNLESS Serbia and Kosovo sign a treaty demarking new boundaries. See how it is done?
That said - SHOULD you want to discuss unilateral secession (of Palestine) from a parent State (Israel) without the assent of the parent State (and you don't, you really, really don't) that would be fun.
First off all, Egypt and Israel have both signed a peace-treaty and as such recognized each others national borders - based onto the previous Green Line (in turn nothing else but the previous established British Mandate borders) - the same goes for Jordan and Israel.
Israel and Egypt signed a peace treaty which CONFIRMS the MANDATE borders for Palestine as being ISRAEL. (Same goes for Jordan). The borders of the previous administration. There is NO MENTION of the 1949 ADL, except to remove any claim Egypt (and Jordan) have on ISRAEL's territory within the borders of the Mandate for Palestine.
Why is this so freaking hard to understand?
Prime minister Begin and the Israeli Knesset agreed that a transitional self-governing Palestinian authority was to be elected to replace Israeli political and military forces in the occupied territories of Gaza and the West-bank.
Finally! Wow. Took you long enough to get here instead of talking about twelve other irrelevant countries, and the irrelevant 1949 ADL. Yes! Israel AGREED to a transitional self-governing Palestinian authority. I'm pretty darn sure, I've already mentioned this. Do you know what that does? It makes legally binding boundaries between the parties who agreed to those boundaries. You know what those boundaries are? We should be talking about them. It also creates a legally binding agreement that the final boundaries between the two States (once Palestine becomes one) are to be negotiated in a treaty. Both parties agreed that they are NOT going to accept the previous administration's boundaries, and instead will negotiate a new boundary via treaty.
It is only Israel that keeps on defying UN resolutions, and treaties and refuses to hand over Gaza and the West-bank (clearly defined territory) via the previous known and acknowledged Green Line) to the Palestinian authorities, and recognizing these territories to form a sovereign Palestinian State.
Wait what?! You have a
treaty between Israel and the government of Palestine which delineates final boundaries between them?! Whoa. Show me.