For starters, Russia will never agree to give up Crimea. And Ukraine can’t push them out. And it is doubtful they will leave the Donbas. Russia shares a 1,500 mile border with Ukraine. They will be neighbors for the foreseeable future. Time is on Russia’s side.
Ukraine must agree not to seek NATO membership in return for security guarantees.
Doesn’t matter if Russia “agrees” or doesn’t — it didn’t ask for permission from Ukraine before grabbing Crimea.
Russia can collapse - no empire lasts forever — the Tsarist empire collapsed and the USSR was smaller. The USSR collapsed and the Russian Federation was smaller.