I see your point, that Truman wouldn’t know it’s weird (and true, he may not) but it’s just weird enough for him to think about it. Like I said, even if it’s just the thought of “My life is so chaotic with all these new people in it”, that’s a thought that shouldn’t be there. It took him a series of strange events to catalyze the realization in the movie, but it was the thought of that one lady being forced out of the show — and the forced love interest as a result of that — that really got him thinking about his life, and if such an event happened every two months then that’s a lot more clues to piece together.

It would be an unnecessary risk. The Truman Show was the highest rated show by a landslide, so they wouldn’t need guest actors to boost ratings. Besides, the director was introduced as a sort of purist character, an artist, someone who wouldn’t easily stoop so low as introducing chaos to Truman’s life without good reason for it, especially if it would make him come closer to finding out the truth