I'd never thought about it, but it doesn't make much sense, even for Miss Keith.

Maybe Lawrie was left on the team as a character-building exercise in the same way that Marie was, but wasn't thought reliable enough for a part in the Play? At this point Lawrie's only acted in The Prince And The Pauper, which there was little staff involvement with, so they don't know that her usual flightiness and tearfulness doesn't apply when there's a play in question.