I think you are probably overthinking this. At such low temperatures, you're not going to harm it by leaving it too long in a dehydrator. All it can do is get dry. The first "overnight" seems to just be to make it easier to peel, which probably isn't even really necessary. So 8 hours, 12 hours, probably doesn't matter. And then I think she probably meant 6-7 continuous days in the dehydrator, but it might not take that long depending on how hot your dehydrator is, what the airflow is like, how much garlic you're drying, etc. Just take a slice out every so often when you think it might be done, let it cool to room temperature, and see if it is perfectly crisp or if it still has some bend to it. If you go longer than you need to, you haven't harmed anything.
I think you are probably overthinking this. At such low temperatures, you're not going to harm it by leaving it too long in a dehydrator. All it can do is get dry. The first "overnight" seems to just be to make it easier to peel, which probably isn't even really necessary. So 8 hours, 12 hours, probably doesn't matter. And then I think she probably meant 6-7 continuous days in the dehydrator, but it might not take that long depending on how hot your dehydrator is, what the airflow is like, how much garlic you're drying, etc. Just take a slice out every so often when you think it might be done, let it cool to room temperature, and see if it is perfectly crisp or if it still has some bend to it. If you go longer than you need to, you haven't harmed anything.