I have the same telescope & reducer. After a few months of fiddling with the back-focus, I had the same issues... imperfect corner stars, and one corner that was usually worse than the others. It helps a lot if you aren't using any adapters that have short threads. Nice, long threads reduce the chances of any flexure or tilt of the camera due to imperfect machining of the threads.
When I moved the camera to my C11 recently, I was surprised to see that some of the problems with star shapes came along for the ride. The culprit was a short adapter (4 mm) that was still attached directly to the camera face plate, which I failed to remove. It had very short threads. I now think that adapter caused some of the optical issues with the Esprit as well.
I still haven't gotten the Esprit + reducer combination perfect, though I now have some hope after realizing that one of the adapters didn't work with my familiar C11 either. I did produce some nice images with the Esprit + reducer over the winter, but if you insist on perfect stars in the corners, you wouldn't be completely happy with them.
FWIW, your star shapes in the corners look the same as mine did when the camera sensor was too close to the optics. I think you need to add a little bit more spacing and the corner stars will improve.