I'm not actually sure Mab agreed to the condition not to order him to kill any loved ones. She said the Knight's job is to kill who he's told to kill, and then they moved on to discussing other clauses without actually resolving that disagreement. I figure she just knows giving that order would be an immediate path into him turning into a deliberately useless mediocre knight, so she wouldn't go there unless she absolutely had to (e.g. if someone Harry cared about was Nemfected).
As for his paralysis returning when he disregarded winter law, that doesn't necessarily mean she welched on the promise to outright fix him. Even if she did grant him a 100% good-as-new, lasting fix, she also didn't promise not to paralyze him again later as a means of discipline.