To be clear, once you lose once and see the scene, your next encounter with any of the regular enemies should trigger a one-time dialogue at the beginning of battle. I just double-checked and it is working as intended as far as I can see, so I'm not sure what could be causing your issue. If you like, you can try taking that first loss again from an older save to see if it goes through this time. There's no extra art though, it's just a bit of banter.
Not sure I fully follow you here. The trigger is obviously not full power at first, as evidenced by the move often failing in combat. Once Mezz reaches a certain point it becomes trivial to use it to disable him immediately, which is what the boars do.