1. I cannot say with 100% certainty that this plugin works with ClassiPress as we do not officially support third party plugins. I am aware of customers who have mentioned using it, however they have either reported issues, or have had to do some tweaking themselves to get it to work.
2. Yes you can choose to ad or remove widgets as you wish, and there are 6 different areas you can configure from the widgets screen: Main Sidebar, Ad Sidebar, Page Sidebar, Blog Sidebar, User Sidebar, and Footer.
3. Yes you could certainly do this, although some customization would be required on your part to do exactly what you describe.
4. You could achieve such a thing using a
multisite setup with some amount of customization. fleabay.net is a great example of how this can be done.