Sorry, I misread your post.
It doesn’t violate CoE, it just draws energy from outside the ship from hyperspace. That’s not a violation of CoE any more than a sail or solar panel is. And normally all that energy goes into maintaining the wedge and moving it; it doesn’t actually cost them a significant amount of energy to accelerate, the wedge does all the work. In a grav wave in hyperspace though they get enough energy that they can just shut off the fusion plants and draw power from the sail.
As for using it for power elsewhere, I expect they just don’t bother. With their version of fusion power they get plenty of cheap energy out a neat, compact power plant without some bulky & elaborate method of getting energy indirectly from an impeller.
The Graysons were stuck with fission power plants before their alliance with Manticore. Tying an impeller-driven starship to a rotary generator would have been a Godsend for their power needs.
Slight amendment - in order to accelerate a starship, all you need to do is create a gravity field at the front. The gravity field does all the acceleration of both the ship and the crew, thus avoiding any squish (I suppose you would need to have an extra 1 g gravity “down” for comfort).