Actually, no, the coal plants don't get shut down to "keep the balance right" if "the wind didn't blow for a week". CO2 emissions are a long term problem, not a short term conventional air pollution problem where the particle count of soot, VOCs, SO2, etc, are making a local smog problem that the EPA is upset about and the schools are cancelling sport activities because they want the kids to breath as little as possible for a few days.
"Spare The Air" days and global warming are not closely related.

The low carbon rule might have to be averaged over a year. However if say the requirement was 20% and the utility was close to that near the end of the year they may not risk fines if the wind decreased.

There seem to be plenty of reasons to lapse back to coal
- bad people control the gas pipeline
- drought dried up hydro dams
- heatwave/cold snap increased demand
- system component failure.
A low carbon rule should offer few excuses. If the utility can't make its own green electricity then it should buy it for resale.

You are right, they really could make a special law that capped coal at a max of 20% (to minimize greenhouse warming) and a minimum of 19% (to preserve jobs for coal miners), so that people would suffer a 10,000% tax on coal based electricity if it went over 20% exactly.
Perhaps they will. We've done other stupid things in the past.
I'd like to say you were a troll looking to make nitpicking objections, but I know too much history to say that you are wrong. We've done things that were stupider. Trust me, we've done things that were way, way, stupider.