Ok, here's my take on the whole situation:
This idea is great, and sorely needed in what has definitely evolved into a Flash community. I know that there is a great deal of collaboration going on, since many people are great artists, but can't script and vice-verse, and because sometimes a project is just too big for one person. I think this will also encourage more people to collaborate, since they can now get official credit for what they do.
I think that, if there are multiple authors, it should list the submitter only on the movie's page, and then have a link below that labelled "Co-authors" or some such, and clicking that would open a new window listing all of the co-authors. This allows you to have many without making a mess of the layout.
I don't think that there should be too low of a limit on the number of co-authors, but I suppose 5 would be a good number. I mean, if someone just drew a background for you, they shouldn't be getting co-author.
The idea for an approved collaborator list is definitely necessary in order to prevent collaboration spamming.
As for how a collaborator could be added to a movie submission, I'd suggest that you'd submit the movie with a checkbox labelled "Collaboration" checked off, and then after you've filled out all of the other submission information and clicked "Submit" you get a dialog that would allow to type in an NG User's name, or select from a dropdown the name of someone you've collaborated with in the past, and click "Add Collaborator". You'd repeat this for each collaborator until the list was done, and then actually Submit the movie to the portal.
As for awards, all co-authors should receive them. Abuse would be limited under the logic that the people who usually win these awards are not the people prone to abusing the system. And for user ratings, it should affect all the authors' ratings equally. If you're going to put your name to something, you're accepting responsibility for it.
You should not be able to add/remove co-authors after a certain amount of votes on the movie. This prevents people from taking their names off of bad Flash they submitted to bring their average up afterwards, and prevents people from bribing others into adding their names later on to a highly-rated submission to bring their average up, or get awards on their account.