If they are removing people from the flight (as opposed to just changing people’s seats) the plane is too heavy to takeoff safely in current weather conditions. Usually that means it’s too hot outside.

I was in the middle of the response when I remembered that it’s a common enough question that I can just link the article that will explain it better than me.

https://simpleflying.com/aircraft-weight-and-balance/

Don’t give them ideas.

I think that’s pretty standard. When I flew as an unaccompanied minor, ages ago, I’m pretty sure we had to pay a little extra because the airline is required to supervise the minor.