In answer to your questions:
1. In terms of customizing the default theme, yes it can definitely be done as there is no encryption which would prevent you from doing so, and we absolutely recommend the use of a use of a child theme in order to retain those modifications in the process of an update. You can find useful information relating to this in the
AppThemes developer center.
2. There is no specific image number setting in the Vantage admin dashboard, and the default number attached to listings is 5 images. You'd need to modify code to change this as the custom form builder does not include image options.
3. Version 1.2 of Vantage will allow multiple category selection (this is not an option in the current stable version or demo site). Vantage v1.2 is currently in the public beta testing stage so is due for release soon. The sub category drop down appears under the "American category option and you'll see see from the
demo site when you look at the American listings, that those sub categories appear as "Listed in", but are not additional to, as such. Thanks.