I am a professional not-carer.

We're gonna be good, right?

We're gonna keep our phones off and our mouths closed during the showing, right?

We're not gonna take pictures or videos of the screen, right?

We're gonna take our cosplay headgear off so we're not blocking the people behind us, right?

We're not gonna loudly discuss spoilers in the lobby around people who are on their way in, right?

We're gonna keep online discussion to designated, spoiler-safe threads and servers, right?

We're gonna be good, right?

We're gonna keep our phones off and our mouths closed during the showing, right?

We're not gonna take pictures or videos of the screen, right?

We're gonna take our cosplay headgear off so we're not blocking the people behind us, right?

We're not gonna loudly discuss spoilers in the lobby around people who are on their way in, right?

We're gonna keep online discussion to designated, spoiler-safe threads and servers, right?

Answering the second question mostly, because yeah, I think completing the pacifist route is what would count as the "win" condition for Undertale. Both the game itself and Toby in follow-up statements have made it clear that that's the intended place that you're supposed to end the game.

This also seems to be how the Gameoverse operates on a meta level: Once the game is over and there's no more story to tell, the world stops existing, almost like a theoretical player is putting the game down and never playing it again.