The Old Mixer, it was an internal matter inasmuch as the legitimate government catching terrorists is an internal matter.
Also, it was NOT a romulan invasion. It was romulan help given to one faction in the civil law which, while having no legitimacy, was nevertheless given legitimacy by the PD.
The PD most definitely prevents the federation from protecting its interests against enemy action - Redemption is clear on this point.
And only the romulans artificially withdrawing at the end can give the illusion of the contrary. The federation's actions, without the writer's support (which alone dictated the romulan withdrawal), were utterly useless.
BTW, the neutral zone is between the federation and the romulans, not between the klingons and the romulans. And during TNG the romulans crossed the neutral zone every other tuesday with impunity.
You like to imagine an extreme interpretation of the PD that obviously wasn't exercised in the examples that you cite.
You'll have to come with something beyond the slogans without support from the episodes to convincingly argue that the PD was not insanely applied in those episodes.