There is a very big company that seems to hide out the "ads by google" whether or not they gained permission I don't know...
They're probally a premium publisher or what ever google calls users that get more than I think 20 million hits on a site. And changing ads by google to something else is very much against TOS.
No, you cannot. You may be able to do so if your site conforms to the requirements needed for premium publisher status. Even then, you will need to seek advice from Google on what is and isn't possible.
Ouch, 20 million hits! Makes sense they make about $300,000 per month selling products let alone google adsense lol
you are not allowed to edit any part of adsense codes. the example that you mention is against the adsense tos