On that question of going back on one's word and the usefulness of agreements, let me just say that from a Russian point of view, there was a promise made at the reunification of Germany that NATO would not move one inch east. That promise has been broken, with serious security implications.
To the question of where a UN mission—if there is going to be one—would be in the Donbass, whether it would be on the border between Russia and Ukraine or along the line of conflict, I should think that can be sorted out. It can be both.
It's important to note that the Russians don't object to and aren't trying to change that border between Russia and Ukraine. You remember that the rebels asked the Russians, “Please, take us in as you did Crimea,” and the Kremlin and Putin said no. Why? Because their internal interest in the geopolitical orientation of Ukraine is related to all of Ukraine. Ukraine wasn't invaded twice through the Black Sea and Crimea, so the Russians don't want to change the border between the Donbass and Russia.