1) The process leaves a node over the default transition automatically only if there is no node action. If there is a node action, then it is responsible for explicitly signaling. In your example, if the three actions are executed in three different nodes, then you do have to signal in each case.
2) Every process definition could be used as a subprocess. In the example of section 7.7, the "interview" process was defined and deployed separately, then used from the "hire" process.
3) Never. The code that uses the jBPM objects is responsible for managing transactions and saving objects. This is deliberate. You can choose to persist your objects internally, within actions, or externally, in the code that provides signals (e.g. servlets, session beans). With the latter approach you can use jBPM in CMT environments.
4) Can you pinpoint the specific mention?