Hi Hui,
Let's go straight to the answers:
1) the migration project, now included in jbpm5 is only for process definitions not fo the API usage
2) Yes, you will probably need to do it manually, and it really depends on what you are doing with jBPM3, because there are some not mappeable methods
3) I'm not sure about this, you can probably get the WorkflowProcessInstance, get the Process definition with some getProcess() method or something similar and then get the start and end node. I'm not sure what do you want to do that, but I think you can do it in some way, it should be so difficult.
Hope it helps!