Well, what about replacing the huge amount of temporary hit points with a reasonable amount of Damage Reduction and Energy Resistance? DR X/-- equal to his class level +1/2 his non-Defender class levels. 1/2 his character level? Energy Resistance against all types of energy equal to the same, or against one type of energy equal to 5 times that amount (whenever he enters the stance he chooses the type)? That would make him fare better against the Dragon, but also maybe slightly better against the traps, and probably no change to the other encounters.
Honestly I still say he does draw the dragon. You say the dragon might ambush him. I say, he's level 15. If he doesn't have boots of flying he's still got a party, he's just going to be wasting their daily resources rather than a fraction of his large sum of WBL. A Dwarven Defender is never going to be ambushed by a dragon while he's by himself in a real game, but even if the DM's a jerk and the Defender is, and he can't fly, the Defender isn't going to stick around and test his strength. He'll alert the rest of his party.
But then again, this is the problem with this test. It just devolves into a bunch of "what if" scenarios, and since, as you say, anyone can shore up any weakness with magic items it doesn't work to solve anything. DnD is a game with more than 1 player, and where every gameplay experience is different.