Question 1 - FOR PPC YOUR SITE IS READ ONCE YOU SUBMIT YOUR ADS, when you enter your FIRST ADD directly after you enter keywords. your site is read, when you see those 3 google balls rolling across the screen that is when your site is read, IT IS READ 1 TIME AND THAT IS YOUR QUALITY RATING, then you get either allactive keywords or some are inactive based on quality rating, so if you have keyords inactive A - make ore relevant ads, b - bid the keywords at the minimum bid for a week while you build up a solid CTR, you will lose money during this, but with a high ctr your cpc will drop thus converting later on or as it has worked in my niche) , drop the keyword that is deemed irrelevant, or target it better, once it is scanned 1 time you are screwed unless you do the following above, and it will take a week or 2 to trickle back in and bring those that are inactive back to inactive.
Question 2 - no you cannot drop position later at night, what you can do is set a time in which your ads would turn off, then set the time for them to turn back on. BUT EVERYONE has lower converting traffic at the wee hours in the morning, but id rather still be showing 23/7 to build a brand presence, but if you dont really need to be branding then id do the above and set the auto turn off times. hope that helped ....