What should I bear in mind regarding my speaking pace and how I conclude my presentation at the assessment centre?

It’s better to speak a little more slowly than too quickly, and make good use of the art of the pause. Please do not end your presentation with the words: “Right, that’s it.” It goes down much better if you thank the audience for their attention.

Can the other candidates also be present during the role-play exercise at the assessment centre?

That is rather unlikely. For one thing, the tasks would have to be varied frequently enough; for another, the participants present might also draw conclusions for their own conversations from their observations.