New Post: How to Query historical task log with any conditions and and how to list instances of a task

1) Look at the CorrelatedTaskEvent class in the TaskSchedulerEditor library. I believe it meets your requirements. If not, the code for that class should give you some ideas on how to pull the history from the EventLog using your code.
2) The GetInstances method only gets running instances. For historical instances, you will have to pull them from the EventLog.