about waiting in between pieces... I don't think it's necessary. I didn't, at least. The judges were very respectful, and they were sitting there quietly. I think you should judge the silence yourself. Relax for thirty seconds, collect your thoughts for the next piece, then start.
I don't think they will tell you when to start unless you look at them expectantly, because they probably will want you to start whenever you are ready. Of course, if you want, you can always look at them blankly, then they'll instruct you.
I agree on the sightreading insights. I was asked to sightread as well, and like she said, don't stop unless they tell you to. Pretend you are confident, even if you mess up badly. Smile, be amiable, show them positive attitudes, then you will certainly gain some favorable points.