How long do meatballs in sauce need to be heated in a crock pot?
They're meatballs.  It's a crockpot.  Why bother refrigerating them or cooking them in the oven?  Just brown them in the oven under broil and then set the crockpot on low and cook them overnight in the sauce.  They'll be perfectly done, perfectly safe, and delicious in the morning.  A few extra hours won't hurt them at all and then you won't have to worry about time, temperature and food safety.