I will try to answer these in the order you posted them.
1) The PowerCMS can be a part of your site admin, but since it is just creating page content it would not be the entire admin. I would suggest that you use PowerCMS and keep it separate from the other portions of your admin site. The Security in use comes from Security Assist so it will work with this.
2) Like the first one, you can integrate PowerCMS into the admin area of your site, but it will just be used to update page content and will not interfere with other aspects of your admin. You can use Security Assist to secure all of the pages in your admin area.
3) You can configure the Authenticate user server behavior on the login page to work with your existing id column in your users table. In order to do that part though you would need to determine what id corresponds to what users.
4) It would be possible to integrate some server behaviors of Security Assist and DataAssist with the sample store but I would not recommend it. I think it would be more time consuming and less flexible than if you created your own. Since PowerCMS just handles the page content you can use that with any site or store.
5) Having the multiple items with a single add to cart is not a feature of eCart. You could build something like this but it would be a custom implementation. We do not have any documentation for this currently.
What you should keep in mind is that the tools will work together, but they are used one at a time in a modular fashion. This is how you should make use of the tools, you should add in the functionality from one tool at a time to build up your site.
