How do you know that “Kevin” and Thorax aren’t actually the same chengeling? The episode kind of implied that Thorax didn’t head towards the Crystal Empire until after Flurry Heart was born, before that he could have just been wondering all around Equestria trying to find a place he would fit in. By the time of Cranky and Matilda’s wedding he could have been just passing through Ponyville, which would also explain why we never saw a changeling in Ponyville before or after that episode.