I think, Richard, that we're both inadvertently arguing the same side of the argument, and not realizing it.
Previously, it sounded like you were simply saying, "They can't do it."
But, what you seem to really mean is, "The current supernatural political situation is such that it makes it extremely improbable that they would succeed, and even if they could there may not any advantages for doing it." That I can agree with, but it's also a very different statement from "They can't."
The thing is, when a new country comes into being it's with international good will. When South Sudan broke away from Sudan you didn't have the US, Russia, China, and various European powers calling Sudan a wimp for letting it happen. You didn't see them circling like sharks on a blood trail, working out how they could occupy and annex both nations.
Palestine's current situation regarding the U.N. might be a more apt comparison to this scenario (though, with unintended and unfortunate correlations to the factions within the novels)... Without delving further into politics, look up "Palenstine 194" for more info.
To make a long story short, it's unlikely to actually happen, but Palestine is submitting the application as a polical statement and to make its allies and enemies commit to a stand one way or the other, and to force the issue past stalled negotiations.
There U.N members that recognize Palestine as a state, but do not support their membership. There are members that don't yet recognize them as a state, but support the case for membership. And each member has their own reasons for doing so.
Hilarity ensues.
But let's look at this from another angle - what would any other group of mortal spell casters gain from signing the Accords? They are already "protected" from supernatural predators by the White Council being members so what would be the benefit?
Independence and autonomy from the White Council and their laws.
And don't say "protection from the White Council". The Accords regulate the peaceful interactions between the nations and help define how wars are fought - the new group wouldn't be protected from the White Council any more than the Red Court Vampires were protected from the White Council declaring war on them.
You're right, but...
If successful, the new group would now have a recourse for lawful retaliation and recompense from the White Council, when the White Council, for example, sends Wardens to summarily execute a Warlock for breaking one of the Seven Laws (a terribly good reason for the White Council to oppose the proposition).***
The Warlocks would cease to be a troublesome faction of the White Council that the White Council could deal with in any manner they see fit, and become an independent power that's a rival and an equal within the Accords, who must now be treated with diplomatic respect, because they can openly have allies of their own to help defend them from the White Council.
***Which brings up a larger issue... Just what is the White Council's jurisdiction with regards to the Seven Laws? They seem to have assumed authority over all mortal magic, at least, but what if someone or some group decides to challenge that authority?