Dave,
The 2/rev vibration may occur with pitch change also as you suggest. But should we care. If the pitch change or roll is done slowly, the vibration would be minimal.
The advantage of restricting the flapping with a rigid head would be worth the effort in spite of the additional issues. For a coaxial or intermesh the rigid head would be best I think, to avoid blade contact.
My point is:
The models work very well with rigid hubs. I dont see any vibration. The vibration would be hard to see on a model, I admit, but I certainly see other types of vibrations in my model. A model cannot be compared with a full size in some respects of course, but the models show the way for simpler design.