As a small tip, even if you are allowed to place that many, many webmasters including me have found that sometimes less is better due to the fact that if you place 20 ads the chances for someone to click less paying ads are higher then if you have only 5 ads on the page for example. Just test and see what's better for you.
Yes. I also read from one blog that less is better. Having only 1-2 adsenses on the page should bring CTR up.
I haven't put much thought into that but I'm pretty sure you're right. I did some work to one of my sites and ended up removing some ads and my earnings went up. I never even thought about that being the reason. Good detective work.