"I hate to say 'no' to all your questions, but I kind of have to. There may be other methods to accomplish your goals though.

1. The thread word limit cannot be increased. Not sure what the assignment is, but could you not have the students post their responses on a page instead? If you describe the assignment that this would be necessary for, maybe I can help you find an alternative method.
2. Posting blindly is kind of counteractive to the 'wiki' concept, so this is not possible. I assume you don't want students to be able to copy other students' work before they discuss the assignment? Maybe there can be a 10 minute window just before the assignment is due for posting. This should prevent copying due to time constraints. Or you could have them e-mail their responses to you and you can post them all at once.
3. Color coding does not function in threads, but the names appear so you will know who made posts. As for edits on the page, auto color coding cannot be done. I would assign each student a color that they would be required to use.

Once again, sorry for all the disappointing responses. I hope the suggestions helped anyway."