WD-40's carrier evaporates. If you have something like PB-Blaster, it may help to soak it in that stuff for a while (or any other Liquid Wrench type stuff.)
Other things you can try: Heat the snot out of the area (make sure there aren't any other plastic or rubber parts nearby that will be harmed), then freeze the plug remainder (spray freeze should be available at an electronics shop - if not, a can of Dust-Off turned upside-down will work, too), then try an appropriately sized EZ out. The cooling may shrink the part enough to break the corrosion.
On this side of the pond, some people swear by melting candle wax onto frozen fasteners and heating the area to get the wax to lube things up. This trick has never worked for me, but maybe it will for you?
You may have to resort to drilling the wall thickness of the stubs down, and picking the threads out. I would use this as a last resort.
More experienced mechanics may have better options for you.