My experience has been that my outdoor plants will tolerate frost on the leaves and temps down into the upper 20s. Below that was a different story.
Cool days, like when it is in the 40s or lower then the plant does slow down and waits for more sun and warmer temperatures.
A lot of outdoor growers mention doing this on those nights when they expect frost. It works but the tarp should come off the next morning after it starts to warm up, especially a plastic tarp that is laying directly on the plants. You can leave a plastic tarp over the plants if the tarp is on a frame.
Yes. Yes, it does. If light was not needed once flowering started then there would be no need for indoor growers to continue to have their lights come on for 12 hours each day for 8 or more weeks. That light shining is what promotes the photosynthesis that helps the plant produce the sugars, hormones and everything else it needs to continue to grow the flowers.