Good post Gunship.
I think a big part of the problem may be that when the MEC gets into these closed sessions they seem to act just like the government or the military. They tend to over-classify everything. When the doors are closed and the negotiators are in the room then just as a routine matter they stamp "CONFIDENTIAL" on every piece of paper that flows through the system whether it really needs it or not.

Somehow this notion of using split duty periods as a bargaining chip became part of the confidential "direction" they give to the negotiators. And from that moment on the MEC reps just assumed that meant they weren't supposed to talk about it.

Clearly some of that "direction" has to remain secret. Let's call it the "tactical" stuff. But there was no good reason not to tell the line pilots that CDOs were being considered. There was nothing to be gained by keeping that a secret.

I think the MEC just sort of let institutional inertia overcome common sense.

That's one factor anyway. There are clearly others.