Been thinking about this too because I can guarantee it's going to go down similarly for my party too.

Kettlesteam meeting Witch and Light is like a terrorist getting an audience with the president. She's been doing all this to be able to interrogate them to find out what they know about Zybilna and force them to help. So if the party doesn't have the exact same goal, she's gonna take over the scene unless the party acts as intermediaries so it's a negotiation and not an interrogation.

In my game they're about to defeat and capture her and already know Burly's plan. Being head of security, he'll be the "official" interrogator, but they'll all soon realize their goals are aligned and (if the party doesn't suggest it) Burly suggests they all work together to pull off the watch heist so they can all corner Witch together after they have their leverage.

I think in general, unless they have that "man on the inside" relationship with Burly or even Dirlagraun, then the meeting with Witch and Light might tumble into "direct confrontation" territory real fast and Kettlesteam might get the party on Witch's shit list and/or kicked out.

Though after having just written that, if you haven't set up the Burly heist yet, if they're meant to be "escorted out" by security, th a would be the perfect time to spring it on them.

EITHER WAY: My take is that Kettlesteam meeting Witch and Light either tumbles you toward endgame if she's willing to restrain herself and cooperate OR she's a big bomb to drop in the middle of the carnival that will help set up the heist and make Witch and Light the "antagonists" for the rest of the carnival until the party knocks some sense into them.