Re: Dawn will be orbiting Ceres for a very long time
As you said, the hydrazine thrusters and reaction wheels are purely for orientation; the ion engine would be used to change the orbit.
The loss of the reaction wheels, while a serious issue that requires careful mission management, is hardly a show-stopper. At worse mission control could put Dawn into a slow rotation that matches the orbital period around Ceres; this would minimize the amount of re-orientation that needs to be performed, and hence allow the hydrazine to last that bit longer.
This sort of slow rotation is commonly used on Earth orbiting three-axis stabilised satellites, and was first used on a deep space mission by Voyager 2, so its pretty much a standard manoeuvre.