Add easily your own copyright at the footer of your RSS feed posts. It improves the SEO and avoids content scrapers to steal your posts.
No. The plugin will just add a copyright message on your RSS/ATOM feed. Malicious people can still copy/paste your post and scrap your RSS feed. However, a splog (spam-blog) that copy your posts using your feed will normally automatically copy the copyright message too.
Yes, it is possible. Add a specific message, like an unknown words in your copyright message. Then, wait few weeks and search this word in a popular search engine like Google. All websites using this unknown word may be scapping your content.
Yes and no. Google avoid to present the same content in his results pages. It tries to determine who is the original author for any content. Without any links, Google may estimate that the bigger website with a higher notoriety is the original author. If the splogs copies the copyright message with the link to your original post, Google may understand that you are the first one who published the content. But this is not a 100% sure.