How can I manage Intent association with Apps?
I believe it's a no to both those questions. Yes you can clear default associations only if you know which app is associated with with Intent (you will still be prompted with the dialog asking you which app you'd like to use with the current Intent). 

Apart from that there is no way to clear associations. They are hard coded inside each app, there is no way to remove them.

UPDATE: To answer your edited question head to Settings > Applications > Manage applications find your application that you chose as your default app for that intent, click Clear defaults