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)
The Violet Citadel is the location of the Grand Library. The Violet Citadel also contains the Arcane Vault which houses Dalaran's most powerful artifacts. It was from there that Teron Gorefiend stole the Eye of Dalaran and killed Sathera.