I find myself wondering if Deathwing didn't spend *all* his time convalescing in Deepholm, but rather returned periodically to Azeroth, and specifically the Dragon Isles, to set up future machinations and experiments during his long recovery. Some of the stuff we find in the Isles don't seem like the work of the Aspect Neltharion but rather the designs of the insane Deathwing - such as the mind control device, or the long stasis of the dracthyr themselves. It seems quite possible to me that Deathwing might've gone to Deepholm to slumber and recover, returning to the Isles periodically to check on his stuff and perhaps set up some additional wickedness, before growing tired and returning to Deepholm again.