Problem: I have a View of published posts, and I would like to include a button on the front-end of the site to set the post status to be "Draft" instead of "Publish".
Solution: You can do this with a Form that is set to Draft status. Remove all the visible inputs except the Submit button and change the button to say "Draft" or something else.
Problem: I would like to know if it's better to reuse fields across multiple post types, or if it's better to create similar fields specific to each post type.
Solution: It's better to reuse fields if possible, especially if your site uses these fields for filtering Views.