you said "this lady" and not "my manager" or anything like that. Is it possible that this is just another co-worker, trying to throw their weight around?

Also, I know it might seem like everyone left late, because of you, but really, they all left earlier then they would of, if you hadn't been there, picking up a shift you weren't scheduled for, and doing your best!

I've never waitress-ed, before, but I have run that huge silver machine, (to get out of boring study halls in school, working in dietary at a nursing home as my first job, and in a few fast food restaurants). I've always had two racks in front of me - one for glasses and one for plates and trays. Try to put all the plates on one side and all the trays on the other, so that when you take them off, you grab a handful of either one. Soak your silver ware in a pitcher, so you really just have to rinse it off, and toss it onto a rack on the floor. If you're really good, have three pitchers - one for forks, one for spoons, and one for knives.