Hi Nancy,
Yes, you can have Accountings department in the approval flow by defining roles specific to each department and responsible persons for each department can then be mapped with their specific department's role. We may define approval rules for these roles i.e. where and on what conditions these roles should be added in the approval flow.
As far as SLA's are concerned, Ariba Buyer provides a Schedule Task "Escalate Approvables" which defines the period after which an approvables like requisition etc can be escalated to the approver's supervisor if it is left ignored in approver's queue (To Do list). With escalation, the actual approver will be replaced with his immediate supervisor in the system. This will enforce responsible persons to take action as fast as possible. All these escalation actions are also stored in the history of approvables hence easier to track when it was escalated to senior hierarchy.
Amit