How should multiple (related) projects be arranged (structured) and configured so that they can share code, have a related package structure and enable proper unittesting, and ensuring no namespace collisions

[Paddy]

Your second point 'clear separation of "production" code and "test"
code' is best handled by using a revision control system. I know
Clearcase, because I use it at work, but I have heard good things about
Subversion.