We can't rid of newbie right away, but my boss and I went through the schedule and figured out a way that we could phase her out by next week. The trick, my boss decided, would be to do this in such a way that newbie wouldn't catch on that she was being let go until after her final shift. (Ah, the joys of hiring through a temp. agency - they get to be the ones breaking the bad news.)
So that saga will be over in about a week. We still have to find someone to cover the weekend evening shifts, but my boss has changed her mind and is leaning toward staying with temp. agencies for those shifts because they're undesirable hours (4pm-9pm every Saturday and Sunday) and difficult to fill. We'll see!