Thank you for getting back to us.
If you have multiple sites running on one DB you have two options- those sites will either share the content and both see the same content or you can set them up so that they do not share content just the DB then sites will not see each other'c content. There is no option in the middle.
As to your customer's requirement - you are on the correct path to setting this up. Content can be replicated on production with database merges. First you will do the regular ASP.NET push to push custom code from staging to production, then you will take the staging and production DBs and compare them and push changes from staging db to production one.
In Sitefinity 4.0 we will be introducing a syncronization service that will eliminate the manual part of this. It will all be done through SF's interface.
the Telerik team