[@Crimson Raven]The first dome was about six inches, so while tough, not as tough as the second one. Only the second one was infused with necrotic energy. Also the shifts in form are generally low energy, as he doesn't need to feed magical energy to stay in the form, only to shift. As for the undead, while taxing, they kind of got killed pretty quick before the main drain effect(the regeneration) could really get a steady drain going. And the one that didn't die was made from a corpse, meaning that it didn't use up as much energy as the ones he created with just his magic. Add on to all of that the fact that he does have very high levels of magical energy, and I think an estimate of around half is accurate enough. Maybe, a little more actually, like 60% of his reserves are depleted. 

Edit: As for you're other question, than I'd say that it depends on how the necromancer works a bit, but basically, the more altered the corpse, the less capable it will be for the necromancer to control it, especially if it has been created from his own magic. However, Nidhogg would be unable to take control of a corpse that the other necromancer controls, while other necromancers would be able to still effect his corpses, though his more powerful ones would be able to resist or harder to take over. Still, it'd make for a anything from a slight pause to the corpse being completely paralzed by indecision. Only ones immune to this would be the Reborn, which I still have yet to make sheets for.