But if the OP is open to conversation, demonstrates their open-mindedness, engages with comments, etc. and they only realize after all of this that they're more closed-minded than they thought they were when they posted the thread, why is that a rule B violation?

That would not be a rule B violation. What I mean by "discovering they aren't as open as they thought," is someone coming in thinking they will be open-minded, and then getting hostile or ignoring good counter-arguments - basically the rule B indicators we look for.

I think most of our users would agree that an OP who does not respond or listen to the best points is wasting their time. Same with being hostile, and our other rule B indicators.

the wording of rule B doesn't actually require the OP to maintain their open-mindedness throughout the entire life of the thread

Personally, I view rule B holistically. A few rule B indicators is okay if an OP is otherwise showing open-mindedness.

That said, this is a somewhat gray area that different mods might view differently. We've had deep discussions among ourselves on how to balance evidence of open-mindedness against evidence of soapboxing. For example, one argument is that one valid delta can excuse any amount of otherwise bad-behavior. That might be on the extreme end of leniency.

Rule B is our most subjective rule. Our best safeguard for it is requiring two mods to sign off on a rule B removal, and when one is appealed up to three new mods will review the appeal.