I have done something very similar.
What you can do is, like you said, add a user task in process 1 and user a temporary user (tmpuser) to assign to it, since the external user is probably not in user manager.
There are also Events you can leverage as part of LiveCycle. For example, every time a task is created, completed, a reminder is fired, etc. an event is thrown. You can catch those events and act on it. That might be a way to handle them.
As for process 2, you can use the Complete Task service to force the completion of the task and make sure the item moves to the next step of the process.
Let me know if you have more questions.
Jasmin