Let's go straight to your answers :)
1) Basically you can build your own services using the jBPM runtime as you build your applications or you can use the jBPM Console Server, which is provided by the installer. This Server exposes a jBPM runtime via REST. You can also use the Drools Server to expose knowledge sessions via REST and SOAP but that option will require more configurations from your side, or as mentioned before you can build your own custom service that exposes and works as you need for your application.
2) The jBPM Console Server is a web applicaiton, that means that it can be deployed in any servlet container, not matter if it's JBoss or another app server.
3) Once again, as you mention " you imagine an architecture", what you imagine is just one possible way of doing things. jBPM is flexible enough to allow you to do what you imagine, but you will need to set up or build your own components if the ones provided with the project doesn't fulfill your requirements. If you are planning to build an architecture on top of an Application Server, you can deploy the jBPM Console Server, it's not a big deal and you can use that to host your processes, if not you can build your own component to do that.
Cheers