My 2cents:
- Agree, Kamailio training makes your life easier. Specially because even if you know SIP (if you don't that complicates everything), you need to understand how Kamailio works internally, what are dialogs, transactions, branches, etc, the different route blocks, blablabla, what goes where... To be honest, once you understand that, you can do literally whatever you want. If you don't know something, you can ask and someone will give you some help, and when you read it, you will actually understand what whoever is trying to say. Why I'm saying this? Because I could answer your questions to the point, and still you would get a system running but you would have no idea of why or how it works, so what happens when you run into an issue?
- To the point, you probably need different sets of domains and dispatcher groups, and then depending on where the call is going to, route to a specific dispatcher group? This is just an initial suggestion, as you haven't given many details, there are just so many ways of doing things that we would need a little more information on how EXACTLY do you need things to work...