Sigh. For years I've successfully avoided responding to anything I read on ENWorld. It is perhaps appropriate that it was a thread (or two) on paladin oaths that made me decide to break my silence.
First of all, to address the question, I reject simplistic, binary definitions of willingness. Between breaking an oath on a whim, for no discernable reason whatsoever, and being physically incapable of doing what an oath demands due to being physically restrained or unconscious, I see a whole gamut of possibilities, including deception, coercion and duress, judgement calls on which action would result in the greatest good or the least harm, etc. In the same vein, I see an entire range of outcomes and conssquences beyond loss of class abilities or becoming an oathsworn paladin and nothing at all, business as usual, move along, nothing to see here.
That said, I would rely on three principles to navigate this mess of options.
My first principle is that of Justice. Essentially, this means that actions and choices have consequences, and the consequences should be appropriate to the choice or action. I would apply different consequences to the same choice made between the same two options depending on how much freedom the paladin had - whether he was offered any inducement, whether any threats were made, and so on.
My second principle is that of Mercy. This principle recognizes that paladins are fallible and are not expected to constantly live up to an impossible standard. They will make mistakes, and when these happen, they should be setbacks, not game-enders. To me, this means that unless the player deliberately and consciously sets out to do so, there should be no permanent loss or change of class abilities.
My third principle is that of Collaboration. Philosophical arguments are well and good, but at the end of the day, we are playing a game. Whether there are only roleplaying consequences for a paladin character's actions, e.g. dreams, atonement (a period of fasting, extra time spent in prayer, etc. and not just the spell), or even playing the character as if he had a new Bond or Flaw, or there are mechanical implications for the character, this should be acceptable for both the DM and the player. This means that different solutions will exist for different people and different pairs of people.
So, to me, the question is irrelevant. What we should ask is not whether the paladin broke his oath willingly, but how willingly did he break his oath. And then we should follow up with the more important question of what happens next?