Please see the answers below:
1. Stores will share the same domain.
2. Each site has to access different subset of the catalog or have its own.
3. The idea is to create portal for multiple small companies. Among other things companies has to have ability to sell through they site. If it is only one catalog and companies are sharing it we can differentiate company's items by "manufacturer". Companies doesn't have to have merchant account. "Portal" company can be gateway. We do have to do something with shipping in order to tie it with individual company.
I would definitely like to discuss how I could use Mediachase product for this application.
Thanks.
Dragan Kovacevic