I Am not denying grey areas exist. In the case of say Sinn Fein they don't argue that the UK shouldn't exist but rather that it shouldn't still contain Northern Ireland.
No, they're arguing Northern Ireland shouldn't exist. They don't even say the name of the state. Being glib and saying "ah well we got you there, what they're saying is that it *will* exist, just uhhhhh absorbed into another state" doesn’t change the facts. The position of Sinn Fein and other nationalist parties is that the existence of Northern Ireland is a historical mistake and it must end, with the peoples and territory joining another state. Entirely legitimate and mainstream position.
The difference I guess is that a regime can always be changed again whereas it is a lot more difficult to regain the status of a sovereign nation.
Why does that equate to a state's de facto right to exist?
It is one thing to prefer a different government was in place in another country, another to actively attempt to change that government and a whole world different if you seek to obliterate a country.
Nobody is suggesting literally obliterating a country, just that it be returned to the people ethnically cleansed from it, or for it to be replaced by a better and more just solution. In principle it is little different to native people in the Americas saying they'd like their land back. And if you apply the principle of "obliteration" then where does that leave you with Israel's legitimacy? It's a colony of settlers who ethnically cleansed people who lived there (and who continue to do so).
In your view Rhodesia still exists. It doesn't. Rhodesia was a racist white colony and it was replaced by Zimbabwe which has, well, its own problems, run by absolutely different people of a different race and philosophy entirely.
That is a view that needs serious guardrails in place. Saying that a particular country has no right to exist and it doesn't matter what means are used to stop it existing is a very dangerous place to be.
Who is saying the latter part? The Green Party aren't.
The buried lede is of course that Israel is different; people shouldn't be allowed to have opinions like "Israel is problematic" "Israel is an apartheid ethnostate" "Israel demands the consent of the West and its security guarantees to continue to exist" because there are also a load of vile antisemites who will build on this language and treat it as a permissive environment to suggest appalling outcomes. And, there is something in this and it's why I think the Green Party's motions are, to put it diplomatically, "unhelpful".
I will be very happy to allow anyone on the right to have this point about language and permissive environments as long as they apply it consistently to soft bigotries the right has about other minorities. I suspect they won't like that.