I think there must be more to this than meets the eye. If the ACE pump seized up it would shred the drive belt and that would stop the other ancilliaries working, inc the pas pump, but other than that I can't see why the ACE not working would "kill" the car.
As I said, on my previous TD5, the previous owner had a problem with the pump so had it taken off and replaced with an idler, job done.
The ACE is SUPPLEMENTARY to the existing arb's, so the fact that the ACE is not working should not imo harm the car at all.
It sounds to me like you might have been fed a load of poney, by the garage.