1) How does 'defeat' in social or mental combat work, with multiple actors? In my case, I had a pair of PCs trying to tarnish the reputation of an NPC, who responded in kind. The NPC ended up bringing one PC to 0 Reputation, but the 'battle' continued. Can the 0 Reputation character continue to participate in the conflict? I reasoned no, analogous to how it works in physical conflicts and saying that they had no social credibility left with which to monger rumors.
From page 182, when you reach 0 Resilience:
• The stakes established at the Conflict’s outset are applied (see page 168).
• The character may gain a Grave or Mortal Burden, as the attacker prefers (see page 184).
• The character suffers one of three fates, depending on which Resilience was targeted
The first normally means that the PC is out of the Conflict as the Conflict comes to an end. However, if the Conflict continues, then the PC would be able to participate but with significant penalties (-2d and Burdens). It would be very easy to inflict more Burdens on such a PC, suggesting that they withdraw.
2) I must admit that I'm missing something obvious somewhere with how Defense Dice work. Specifically, the distinction between Defense Dice granted from taking Total Defense, versus those granted for taking a diceless action. The latter don't 'appear' until an attack is actually made against them, but I'm not quite sure how that matters.
The reason you don't form dice pools for a diceless action is simply practical as you may not need it. So, the Defence Dice are formed when they are needed.
4) I'm pretty sure that I read somewhere that it is the case, but supposing two characters get into a contest, one beats the other... each character has an Outcome of (Their Roll)-(Opponent's roll), IIRC. Does the losing character also suffer concordant Complications, since their Outcome is guaranteed to be negative unless it was a nudge-broken tie?
That's my understanding.
5) How does grouping Extras work with respect to pools of Action and Defense dice? If one extra is attacked, chooses to spend some dice to defend himself, and survives, that places him lower in the resolution order. Does he still get to attack with the rest of the group for the 'teamwork' roll?
I don't think so. They act seperately.