Yep there certainly is an agenda and if it had zero influence on America they'd probably give it up.Chris and Phil talked about some of the staging tricks (lights and sound) http://content.usatoday.com/communities/idolchatter/post/2010/02/how-to-tell-when-american-idol-is-playing-favorites/1
Performance order is considered a factor in a lot of judged contests and there are certain ways humans perceive things and one of the ways to manipulate perception is performance order.
I do think there are enough reports of song changes late in the game that I'm sure it happens. How much is "we don't want to pay for that song, for you" and how much is "we don't want people to hear you sing that well" I dunno.
However it is hard to treat all your kids equal when you really want one to succeed and others of them fail.