A few thoughts/comments:
1. I'm not aware of a shared database, since most information will be location/country specific, but if any users in your country read this then they may have created something they're happy to share.
2. There's no automatic way of doing this, unfortunately.
3. FrontlineSMS can support multiple devices, so you could have three modems on one computer, one for each network.
4. FrontlineSMS is not designed to work on the network side. It's a relatively low- to medium-volume platform which runs off a standalone computer.
5. I'll leave this for any user to respond to.
6. Same as (5).
Bear in mind when you think through all of this that FrontlineSMS might not be the right solution if you're hoping to build a nationwide high-volume system. It's designed to work at a more local level, off standalone equipment. It would work well for running pilots, or multiple local implementations, but countrywide services would benefit from working directly with operators and using short codes, etc.