1. You can determine in the admin settings what you what to do once the job listing period has expired, that is either leave it published but with an "Expired" message on it, or it can be sent back to draft mode automatically which means it's not published on the site.
2. Depends on what changes you make, ideally you'll create a child theme if you are planning a lot of changes.
3. AppThemes does not offer customization services, however we do have a
certified partners page where you can find developers for hire. Thanks.