I was a bit reluctant to suggest this method as it's borderline butchery but so far your sump plug hasn't responded to the usual methods.
The one i had in mind starts at ~4:15 in this video, using a hammer and cold chisel to "drift" the nut round. It's shown quite well in the video, as well as some other methods that you might want to try.
https://youtu.be/R5d0Bgvjmlk
See what you think, it's worked for me many times, even on wheel nuts that have been fitted with a windy gun aka air impact wrench.
Granted it'll chew your sump plug but the chances are it's fairly well banjaxed already so no great loss.