Not all magic is disabled, obviously, since otherwise the magic swords would turn into normal pieces of metal and whatever materials they are made of. So, Yuki should keep experimenting to see which sorts of magic he still can use. He should be able to freely use all magic that stays inside his body, that is, reinforces himself, since that's what the magic swords essentially are based on. Other than that, it would be up to measuring if the mind-eroded demon lord's ability to disturb magic is higher than Yuki's ability to control magic.