1. Differentiate between the database files and the database engine. In a runtime, FileMaker creates a crippled copy of the FileMaker Pro database engine. It is crippled in that it does not talk out. The database files themselves are given an affinity to that engine but in all other respects are standard FMP files and may be opened with a standard copy of FMP. If you have FM Advanced to create runtimes, you also have the provision to rename files back to .fp7 files.
So, with full copies of FMP 10, you can take these files, enable networking and run them over a network in that way.
2. No - The runtime files DO NOT run over a network as single user files.By this, I mean, you cannot put the runtime on one machine and then open it from another. To do this you are envolking sharing at the operating system level. This is NOT an accepted method of running FileMaker files and will eventually result in loss of data, and/or corruption of data and/or corruption of the files themselves. This is a dangerous practice.You usefull copies of FileMaker Pro on each work station and use Filemaker's native tunneling to network. See the manual or help sections on opening remote files.
You could, one person at a time, using VNC software, use the runtime from another machine. In actual fact, the runtime is still running on it's machine and you are just communicating with the screen from a remote computer.
3. FileMaker, using full copies of FileMaker Pro 10, will serve up to 10 people on a peer to peer network. For more than that you would require FileMaker Server to be serving the files.
4.FileMaker, using one copy of FileMaker Pro, can serve up to 5 people using instant web publishing. This has some design considerations in and of itself. Using both FileMaker Pro Server and Server Advance, you can serve up to 100 people using instant web publishing.
You would need a copy of Pro or Pro Advanced for each user.