How do you top-dress your area/fabric pot with these so close to your plants without killing or disturbing them
Less is more when sreading seeds, but you shouldn't have any issues with killing your plants.
What is the coverage per square foot? How many pounds of seed for 700sq ft?
This varies with how thick of a cover crop you want to lay down as well as if there currently plants growing in your beds. That being said, 5lb. could cover that area really well.
When starting a grow, when is the best time to plant the cover crop seeds?
You can spread the cover crop seed as soon as you plant your clones. If planting seed, maybe wait 10 days to allow the seeds to sprout before you plant your cover crop seed to allow your plants to have a little bit of a head start..