Major issues fitting new drain to bathroom sink
WTF is right! So the uneven sink drain is caused by porcelain chunks? If so they can be smoothed or leveled with a stone bit chucked into a drill. I'm unsure why the drain manufacturer doesn't want putty used with the drain? Putty is a neutral and mild sealer with no abrasive nor corrosive properties. Unless they give a specific reason for not using it, I would suggest it is your only option to make the new drain work. Perhaps that is what the white ring in photo #2 is for; an attempt to smooth and level the uneven drain opening. If it is solid it's epoxy. I see clear silicone in the photos; this must have given someone a headache in the past .