How do I unscrew a broken aerator?
Mineral build up has likely cemented the aerator in place, so:

Wash off the WD40 residue with soap as best you can.

Get a glass or cup. Fill it with vinegar or a citric acid solution, and heat it in the microwave til hot. position the cup under the faucet so that the mineraled up threads are submerged. Let it sit for a few hours periodically warming the liquid and lightly tapping the aerator with a hammer so as to loosen residue.
If you are persistent and lucky, the acid will dissolve the mineral buldup to the point you can unscrew the top half of the aerator.

If you have them, a mototool or detail sander might also be used to mechanically nudge the stuck threads. That'll open channels for the acid to get at the mineral buildup.