Russia has agreed to a European monitoring mission.

I think that Putin is giving an appearance of planning to invade other parts of Ukraine as a threat so that the question will be not "Can Putin keep Crimea" but rather "How to stop Putin from taking more." Then the West and Ukraine will "meet him half way" by letting him keep the Crimea.

If so, this agreement suggests that Putin has decided he has pushed the act far enough, and it is time to make the deal.