1. You should just be able to use the “Excerpt” metabox provided by WordPress. If you don’t see it, click the “Screen Options” toggle at the top right of the edit post page, and make sure “Excerpt” is checked.
2. Not too sure what you mean here – can I see?
3. The load more button inherits the same style as your load more button, which is set in the “Content” tab.
Let me know 🙂