The last story was one of the best examples of "The Test" that I've read since reading "Mr. Benson" How much is a person willing to withstand in order to prove (or discover) that he wants to be someone's slave? And from the other point of view, if you were looking for the perfect slave, wouldn't you keep trying to devise ways that would make them fail in order to satisfy yourself that, while good, this one wasn't "the one" you were searching for? And if they kept passing all the tests, wouldn't you in desperation, knowing that they couldn't be yours, keep upping the ante to find their weakness?
The only thing I found weird was the twist at the end, but that was more because I never saw the previous M/s relationship. It seemed a tad convenient. Seeing some earlier scenes (or at least have him think about them) would have given a comparison between the two experiences and prevented this from being a convenient plot hole filled.