You're welcome :)
1. The Photoshop .psd files are useful for web designers who may be looking to change the layout of the theme, as they contain information relating to the sizes, colours etc used in the default template. Essentially it will save time in having to collate this information yourself. You still need knowledge of .css in order to make use of the files.
2. AppThemes does not officially support third party plugins, so we cannot guarantee that all will be compatible with our themes, but you can certainly try them and if required, make required modifications in order to make them work.
3. There is no encryption which would prevent you from making such changes, so it's definitely possible. We recommend the use of a
child theme when planning changes to the default layout, as it will save you from losing all of your modifications in the process of an update. You can find some nice examples of customizations that other customers have made to Vantage on the
AppThemes customer showcase. Thanks.