Z can not negotiate for any deal. He does and the radicals will kill his ass in a second.
That is most likely why he got his family to hell out of country.
CRZ - Zelenskyy is blocked by the Ukrainian constitution — lines/articles 2nd and 73rd of Constitution of Ukraine cover that exact question.
Article 2 The sovereignty of Ukraine extends throughout its entire territory. Ukraine is a unitary state. The territory of Ukraine within its present border is indivisible and inviolable.
Article 73 Altering the territory of Ukraine is resolved exclusively by an All-Ukrainian referendum.
https://www.refworld.org/pdfid/44a280124.pdf
If Ukraine is to change its borders, it must have an All-Ukrainian referendum. It is also specified what this is in the Constitution, in 72nd article.
Something tells me you are right on the money.