In answer to your questions:
1. The 
JobRoller demo site features all default functionality of the theme without plugins, so no you don't need additional add-ons in order to achieve what it offered there. Add-ons outside of what the default theme offers are able to be purchased in the 
AppThemes marketplace if you wish.
2. Yes, the theme is not encrypted so you are free to make modifications regardless of the license type.
3. JobsApp is a JobRoller child theme, so yes you need an instance of JobRoller installed before you will be able to use JobsApp. It will not function without JobRoller. Thanks.