Yes, a support rep can be associated with multiple portals. When support reps log in into Zoho Support they can find a list of portals they are associated with. They can choose between the portals to work in Zoho Support.
Yes, support agents can be associated to multiple Departments. Goto Setup >Admin Settings >Users >edit the Support Rep and associate departments to the Support rep.
Yes, Support Agent can be associate to multiple Portals. As soon as the Support rep log-in, associated Portal URL's are listed. Support rep can click on a Portal to view/reply tickets.