Rules Questions

That works. (Although I kind of like your idea for a house rule better. )

I gave it a bit more thought since I posted my question, and it occurred to me that when a Lead is in a Contest with a Feature, and the Lead rolls a '1', Watchtower can buy the die for the Trouble pool -- even though the Trouble pool isn't involved in that Contest. It makes sense that it would work reciprocally as well.

So essentially, the Trouble pool is never 'not in play'.

Yes, but one thing I didn't stress. You, as the player, have to 'activate' the Complication. You have to narrate something in the fiction to put that Feature at a disadvantage. If you don't, it's the same as me not buying your Complication. Nothing happens, other than not being able to use that die in the result.

Interfering:
"However, if both sides are committed to continuing the fight, they may each give you a Plot Point to continue despite your Interference. Both sides must buy in, at which point all three of you make one roll each. The two combatants should describe what they’re doing to continue the fight before you describe your character getting in the way, or cowering, or what-have-you.

Compare the results of the two players in the Contest; whoever has the highest result inflicts Stress on the other side. Neither side gets an opportunity to Give In; your Interference has made the Contest about hurting the other side. Additionally, if either of their results is higher than your result, they inflict Stress on you. This may mean you take Stress from both sides!"

Can someone interfere if the contest has already been resolved? Is it then just a new contest? Aina agreed to leave with Mopsa and calm down. Now Walter is starting a new contest to get information?

It's not interfering, it's a new contest. And, Aina agreed to calm down, but not necessarily to leave. I think it works better with the drama for her to stay a spell (see what I did there?).

Gotcha that's how I see it.

We've been doing the Challenging wrong. When you challenge a Value or Relationship statement, you get the three dice over that whole Test or Contest.