In answer to your questions:
1. There is no encryption which would prevent you from changing the layout of the theme, so yes it can definitely be done through css. We recommend the
creation of a child theme so that you don't lose all of your modifications in the process of an update.
2. Knowledge of .php would be useful for making changes to theme functionality, with css for styling. More information can be found in the link above.
3. You can implement your own payment gateway if you wish, however you may also wish to note we offer other
payment gateway plugins in the
AppThemes marketplace.
4. CV/resume information is typed in, rather than uploaded by the job seekers, but the
FXtender plugin does allow uploading. Technically what you describe could be done, but how "easy" this is, is ultimately relative to your level of coding knowledge.
5. Within the theme you can restrict back office access and reCaptcha for new user registration, but in addition to this you could also research other options for increasing security on a WordPress site (rather than specifically relating to the theme).
6. As mentioned above, .php knowledge would be required. You can find some more information on our
AppThemes developer centre page which may help.
7. You can set job plans from within the WordPress admin dashboard with featured listing options available as shown on the
JobRoller admin demo site. Thanks.