This is neither likely nor possible. Poroshenko cannot speak for Western Ukraine - and policies of Right Sector created a lot of people there who see Russians as the enemy.
What is more likely, IMO, is that Putin showed Poroshenko a documentary movie that was filmed in last couple of weeks in the theater of war. And then he showed him an invitation to Hague - a criminal complaint with charge of war crimes. That's the fire that Presidents are playing with. A government has no right to kill citizens just because they have a different political opinion. (The opposite is not true.)
Russia has other ways to apply pressure. For example, it may establish a no-fly zone over the area of the conflict. Precedent-wise, this would be something that the UN will have to accept - it hurts nobody, and it prevents further destruction. Poroshenko is a businessman, not a zealot. He will approach this from the position of logic. Ukraine cannot make people of Donetsk love Ukraine by killing more of them. As the separatists aren't marching on Kiev or doing any other offensive, Ukrainian Army cannot justify its actions that would be fit for Soviet Union in 1968, not for a modern democracy that strives to join EU.
Poroshenko was elected by Western Ukraine. Of course he speaks for them.
Accepting Russian sovereignty over Ukrainian air space and Crimea is accepting Vassal State status.
What a joke! I do not think it is impossible for Putin the mass murderer and thief to be so cynical as to accuse Ukraine of war crimes (they've been doing it since day one), but the idea that Putin has some secret smoking gun is ludicrous. They aren't holding anything back in Russian media, which consists of images of the Ukrainian government with cuts to black and white videos of marching Nazis.