A year late, but for others with similar AC power quality questions:
a] is the problem 24 by 7 by 365, or does it come and go?
I have done about 10 hours of reading about power conditioners over the past several days, in dozens of forums and publication reviews, and regarding your point a), I found it interesting that some have said the system was better in the day, and worse at night, and others the complete opposite: worse in the day and better at night.
I suppose it depends on where you live, and how close your lines are too the other dirty lines and sources.
A good example of better day/worse night is a completely residential area, where the majority of homes have their people not home in the day, but they all return around 4-6 pm and all the A/C systems come on, along with the ovens, TV's, they do laundry etc.
A good example of the opposite is some home near industrial areas, where all the huge power consumption motors and machines essentially stop at the end of the day, and then even better if most homes don't have A/C.
Power conditioning must be the most system dependent of all the components in hifi, so comparisons between Bill in Florida's experience with power conditioner X vs. Y, compared to Mike in Vancouver BC should not be as conclusive as comparisons between speaker cables for example. But if they both claim substantial sound quality improvements I suppose that is pretty conclusive!
