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 planet with the survivors you're looking is named Oltan. Oltan is in the Phontes System which, in turn, is in the Silean Nebula. As for where Dekuuna is, it's the big planet right next door to Oltan. Once you scan Oltan and pick up the Elcor Flotilla, just head back to the Citadel Embassies and talk to the Elcor Ambassador again to finish the quest.