Hi Bing,
1) you don't need to implement persistence, you just need to configure it to work in your environment. if you take a look at the error that you paste here, you will notice that is a JPA error, not a jBPM error.
So you need to check your persistence.xml file and the persistence unit defined inside it. But this is a java question, not specific to jBPM. The error in this case is saying that you have a JPQL (JPA query) which uses the ProcessInstanceInfo and you don't have that entity defined as an entity inside the persistence unit configuration.
2) So, as far as I understand you want to execute an automatic activity, which is writing a file and then a human task to control that right? Then the loop can be implemented with a ServiceTask + a HUman task and the required gateways to draw the loop.
3) If you are using persistent session, you should pass the persisted session ID in order to restore your session and then be able to signal an event. Notice that the event is being signaled at session level, which means that you can do much more things that just signaling a specific process instance.
Cheers