Joy: Cropping the 11 px myself, letterboxing + dithering yields a pixel-perfect result: Nickel doesn't do any more processing, it properly uses my 1080x1429 (the *actual* effective resolution of an H2O) image without any processing.
Since it's PNG & dithered to the right palette: no banding at all, and no extra processing means all that work is not mangled by Nickel

.
Maybe that'll give an incentive for people to actually figure out the exact amount of hidden pixels on their devices, and where they are, because I seem to have been the only one to
care so much about that so far

.