Hello,
I was at Universal (both parks) last Tuesday and Thursday by myself. This principle of "be in line by 10 PM" also applies to the Harry Potter Forbidden Journey ride. I have heard that they keep the ride running each night until around 11:30 PM to accomodate everyone who entered the line by 10 PM. I do not know about the show at Olivander's, but perhaps they have the same policy. Incidentally, I have been wondering if the kids' bedtimes are being messed up by staying out so late, but I assume that they do not mind.