In answer to your questions:
1. Yes, you will receive all theme files with the exception of the Photoshop .psd files, however this does not prevent you from customizing the layout/change colors etc.
2. Yes, there is no encryption which would prevent you from making changes to the theme.
3. It depends on the plugin you use for this, but the one we have on our
recommended plugins page "Social Login" has free and paid options.
4. Customers can edit their ads from the front end customer dashboard, so they do not need to access the WordPress admin dashboard. Thanks.