I think what you have there is a washer and screw shank seized together and the screw shank seized into the sump block's hole. The first thing I'd try is heating the washer with a heat gun, spraying on the penentrant then after a few minutes, trying to pry the washer from the screw. After that is done, you could heat the screw shank, spray on the penetrant around the screw, knock the shank with a hammer then, if there is enough shank, patiently try to unscrew it. Repeat.
Drilling it out will take a couple of reverse drill bits, accuracy, patience, more penetrant or light oil to cool the bit while it's drilling and maybe inserting a helicoil.