Boxing & unboxing is the process of converting a primitive value into an object oriented wrapper class (boxing), or converting a value from an object oriented wrapper class back to the primitive value (unboxing).
The Old Dalaran will stay in Northrend, so yes, mages are going to have 2 portals to dalaran (one to Northrend and one to the Broken Isles)
Rhonin appears in the Wrath of the Lich King expansion as the leader of the Kirin Tor. He is located in the rebuilt and relocated city of Dalaran in Northrend, with Vereesa Windrunner, Archmage Modera, and Archmage Aethas Sunreaver at his side.