c3QReport(MapReduceQueue, true) is a handy way to see all the different Entry types and statuses of those entries in the queue. MapReduceQueue.setPriority() is a great way to priortize certain activies over other within a queue. Standard priority = 300. Lower Priority will be represented by a higher number. And higher priority will be represented by a lower number. Think of it as the place of the entry in a line. The first in line will come first, and the 500th will have a lower priority.
Note that this just sets priority within a given queue. If multiple queues have jobs entries, the platform worker nodes will rotate between the queues when selecting their next job. In order to drain one queue first, you will need to pause() the other queues.