In answer to your questions:
Yes, users can edit and upgrade their listings from within their customer dashboard.
There is a .csv import tool, however this does not include custom fields by default. You can read more about
customizing the importer here though.
Yes, you can
modify Vantage as you describe, there is no encryption which would prevent you from doing so.
Having unique sidebars for each category would also require customization of the default theme on your part. Thanks.
