Hi
For your second question when an agent has not been determined, if you have fixed the root cause and the agent shall now be determined by your rule you can use transaction SWI1_RULE which is recalculating the rules on workitems.
For maintenance :
You can simply create a variant and then run it every night. which means that you don't care anymore who replaces who, because as soon as the maintenance for the determination of the agent has been done (new manager in PPOMA, new cost center owner maintained...)the new agent will be determined and affected the old agent won't have it in his inbox anymore;
Programm of the transaction is RSWIWILS
If the agent determination is not fixed (agent not replaced in the hierarchy, or cost center... whatever you are using) then as workflow administrator you can (if the task allows it) transfer the workitem to another user.
For general information on administration of workflow go into the sap menu under Business Workflow, Developpement, administration and then runtime (or something like it) there are a bunch of usefull transaction.
Cheers
Stephane