You could, and it would be good to at least write about the context of your wiki, what do you expect others to do on it, why did you create it, and such. Don't forget to add a link to your wiki as well.
I see that the last messages there were of a week ago, and these were from the same person. I have removed the threads, but I don't think any other action is needed. Do remember that if it doesn't happen on this wiki, inform staff or other admins about it
Depends on the feature. If you mean the features on Special:WikiFeatures on your wiki, you can use that page. If you mean custom scripts on the Dev Wiki, there's an installation manual on the page of the script. Editing the background of the chat is different though and is not available on that wiki.