This kind of problems are the reason for which we need an overall improvement of the title system in CK2. The game acts like if the nobles wanted to dismantle the kingdoms, but what they wanted most of the times was to keep their privileges and force the king to give more privileges to them. They wanted to keep enjoying the benefits of being part of a kingdom, but acting as independently as possible, to the point of seeking total independence (which happened very, very rarely, but the situation of the Counts of Toulouse before and during the Albigensian Crusade was more or less that one).
Thus, there should be a way for the nobles to avoid what I've seen a thousand times: Half of France becoming vassal to England without the complex matrimonial and military policy made by the great Henry II. They should be aware of the boundaries of the kingdoms. Also, the further they are from the capital, the worse for their loyalty.
And that feature that allows vassals breaking from you with no war, peacefully, is silly.