How do you reach a peace treaty with your own people that simply did not want to follow the newly installed regime? Pro-Russian separatists is a term coined and used to this day for regular Ukrainian people that simply saw an overthrow of their government and did not want to follow the newly installed one. Why not call them "Anti New Ukraine Government" group (sorry haven't thought of a better term). Well the answer is really propaganda and the shaping of opinion of the masses. Because they do not support the newly installed officials, let's associate them with Russia and place the blame in mainstream's heads that it is Russia's fault. I am not exonerating Russia from its involvement, but I see a legitimate self-interest that Russia has with its next door neighbor and strategic view of keeping NATO out of its backyard.
Sorry for my digression, but I honestly do not see a resolution where there will be a re-unified Ukraine. How do you expect to brand, bombard, and slaughter your own people and expect a reunification?