Is one sinful for accompanying someone to a halal restaurant, knowing or predominantly assuming that they will consume alcohol after the meal? If so, is the wage earned thereby forbidden or mixed? And what is the criterion for determining what is permissible and what is forbidden in this situation?

One must think well of Muslims. Transporting customers to places where alcohol is consumed is forbidden if you know or strongly believe they will drink there, because it is assisting in a forbidden act. The fact that they will also eat permissible food does not make it permissible. If the permissible and the forbidden conflict, the side of the forbidden is given precedence. The earnings in this case are unlawful.