Yoma 64: A Kid and Its Mother

What happens if the se'ir ha-mishtaleach (the scapegoat) is disqualified? Whether because it's too young, or has a physical blemish, or because of the circumstances surrounding the day (for example, if the mother goat of the scapegoat were killed on Yom Kippur as well). How this works - or even was discovered - seems a logistical challenge, or few. Also, what happens if you need a new lottery for the goats - which of the 3 remaining goats needs to go to pasture to acquire a blemish to be redeemed (to remove the animal's consecration). Which leads to a machloket between R. Yochanan and Rav... And their reading of the mishnah.