“While the game talks a lot about the challenger having special initiative to really set the terms of a conflict — thereby forcing their opponent to use their weaker stat — there’s no actual way to mandate that.”

There’s no hard mechanic to enforce this, that’s intentional. A clever narrator can try to push for any stat in any situation, provided they can make it sound plausible within the fiction you’ve established. It’s up to the group (and the GM) to arbitrate if they feel a player is getting spammy with their “best stat” in scenes where it wouldn’t work right.

Oddly enough we never had that problem in any of our games. Players chose the stat that made sense for the situation, and didn’t try to come up with excuses for using their better stats in scenes where they didn’t fit. I more often than not saw players pick based on the feel of the moment, knowing that they’re picking answers that call on their lesser stats, and just rolling with it because it told a better story for the moment.

If that is something that becomes a problem in a group, my hope is that it gets resolved outside of the game mechanics.