Yes, you are not the first to point out the black clipping, it was a bit of a compromise artifact in trying to get rid of the colour cast the bad master flat was inducing at the sides.
Markus, the impact on the flats was more or less invisible but on darks taken in daylight it showed up big curved loops of light from various parts of the edge of the frame, the holes were right back behind the primary mirror and there were six of them. I never saved the darks taken that way but I would bet that there would be an arc of light corresponding to each one. Before now I overcame it on darks by just shooting them at night but for sky flats that does not work obviously.
I am still not 100% happy but I reprocessed tonight with a new set of flats I took after finding the light leak, this was just a quick set taken with the garage lights on and the scope pointed at a wall with a couple of layers of white linen over the corrector and with about a dozen layers of gaffer tape over the holes. I could perhaps pull the background down a little more again with curves but I reckon I might stop until I gather more data to smooth the noise down some more.
https://www.astrobin.com/full/4vt2c5/C/?nc=user