RuntimeManager is recommended way to use when processes are involved, for rules itself you don't need RuntimeManager as it's tailored for ease of management in BPM world. When it comes to multiple modules I would say that best would be to have top level module that aggregates all dependencies to other modules and then build up KieContainer for it. Once KieContainer is ready it can be used to get important parts of it:
These two can be consumed by RuntimeManager and the RuntimeEnvironment to provide flexible execution environment. This is the approach that jbpm-kie-services deliver and are fully CDI based.
There has been some work done around RuntimeManager for spring too so might be something to look at. As you already mentioned there is not much documentation yet but soon it will start coming in, meanwhile take a look at kie-spring and kie-aries-blueprint modules of droolsjbpm-integration. Maybe it will bring you something useful if not feel free to open jira for improvements you think are needed.
HTH