Hmm... where to begin...

Let me put it this way: Entire BOOKS have been written to answer questions 1 and 2, and it's still an area of very active research and development

The only question with a short answer is #3: There are a number of open-source game engines that support networking that are worth taking a look at, like Quake 1 and 2. Does Crystal Space do networking? Also, examining the mod tools of companies that are open about their architectures (like the Unreal and Half-Life engines, even if you can't actually run them on a Mac you can read the documentation and understand the concepts). However, I don't think a "real" database (I'm assuming you mean something like MySQL) could be easily adapted to be an action game backend; they're not all that fast compared to most common game engine world design paradigms.