Once per activation...this model may (apply a push or damage). If the "may not" is opted for, then the Woe player has not applied the push or the damage. You can do nothing any number of times. Say, Pandora uses The Box Opens with only one model in range, and that model fails its duel, and gains Stunned. Pandora may choose to apply her once per activation Misery effect (choosing push or damage), or not. If she chooses not to, and then hits her Delay trigger on Self Loathing, giving the model Slow, she can now choose to apply the effects.
If this is not how Pandora is intended to work, so be it, I will accept an FAQ that say so (and quietly, and sadly, put the garbage Woe crew aside until next edition), but the wording doesn't support that the opponent is intended to have total control over the use of Misery. In fact, I submitted numerous battle reports using Pandora in this (and/or very similar) iteration(s), during the beta and never got any push back on using Misery just like this from players or designers when I used it as Ogid and I are describing.