This is my first venture into web site design, although I have been involved in software development and customisation, so I am still learning how to structure your system. I want to be able to develop multiple sites at the same time. At the most basic level I want to have a separate development and production versions of a web site. I realise that you have the ability to have draft facility in the CMS but that is not sufficient. Both content and the structure of each site need to have the capability of being completely independent.
For working on multiple sites (are you talking about on a server or just on your computer?) we normally just have a totally new install for each site. If you want to manage multiple sites via 1 CMS then the subsites module is what you are looking for but this is currently in alpha state and not ready for general release.
The subsites module is more for developing related sites, as I understand it, under one domain. I think a much better option for you is to set up a shared install above your web root and symlink all your sites to sapphire, jsparty, and cms. Then each virtual host just needs a mysite and assets folder. Pretty easy to do.
For staging domains, just set up yourclient.yourdomain.com as a sandbox area and then when you go live you can migrate the changes to yourclient.com. SVN is a big help in managing the versioning.