Hi Ed, I don't really understand your question in the third paragraph of your post, but let's start off step by step for questions in your first two ones.
I think it's best to start off with an example, so if you could show us a simple process you are developing with your workitem configuration and your process bpmn2 I think it would make it a lot easier to discuss how to accomplish what you need and go from there.
I solved the problem I had on how to map a Process level parameter into a WorkItem variable using the org.drools.eclipse plugin propery sheets with the help of folk in another thread. The main problem here is I never saw any documentation on how to do this and nearly all ansers her einthe forum refer to the xml directly, not the designer usage. This means I no longer need to access the session because I could communicate parameters between Process level and WorkItem level without the need to get that data through the session.
Of course at some point I assume I will have to switch between the org.drools.eclipse 5.2.0 Eclipse plugin to the shiny new BPMN2 designer, although it is not clear that this migration will be required, because once again I could not find a direction statement on whether the BPMN2 designer is an alternative designer or is to supercede the org.drools.eclipse 5.2.0 designer.