Hey Claudia! Don't worry about your English - it's great!
To answer your questions:
1. It wouldn't be possible to add tags like you're suggesting right now, although categories work in the same fashiong (except they aren't displayed on posts).
2. You need to edit any post using the duplicated category, and remove the duplicate and add the original. Once you've done this with every post re-publish your whole site again. To view all the posts with a particular category in the editor, click on the category in the editor.
3. Likes are tied to the URL of a page, so as long as the old page URL is the same as the new URL the likes will automatically transfer. If you can't get your old URL structure it wouldn't be possible to transfer the likes, though.
4. You can add a list of recent posts using the RSS Feed Reader element. Drag the RSS feed reader element to your site to add one. To get the feed URL of your blog, right-click the RSS icon in the sidebar of your live site.
5. If you have a blog page on your site already, I don't know why that service would tell you otherwise. It might be a problem with how their software determines if you have a blog.
