Car batteries are significantly larger than powerwall.
Net metering rates are pretty poor, so I would guess not much outside of extreme grid instability. Although, you then might want to selfishly hoard the juice for yourself.
It would not be selfish to run your home off the car battery during peak rates and therefore cutting peak demand on the grid. Everyone wins.
Depends on the nature of the agreement and the need for energy. In a load shedding event, the power company may want to cut useless residential consumers so as to prioritize hospitals. From a maximal good perspective, it is better that the hospital is running than I get to run my AC when the grid is straining.