I grew up in the rust belt, I could get that bolt off!
Before getting started hit it with PB Blaster for a day if you can swing it. Revisit the bolt a few times. You want the penetrant to go between the threads.
As west coast Chris suggested, getting the caliper off will help. I figured it was a lost cause but east coast Chris has a good suggestion. Since you are sending these back as cores, split the caliper. Now you may run into similar issues. Before tightening those caliper bolts whack them with your BFH.
I don’t recommend removing the strut just yet. You want the leverage of the strut in place for turning the stuck bolt.
You will want to buy a decent sized screw extractor, and a set of hardened drill bits meant for metal. You will ideally want a socket that fits the end of your extractor also. I would use one about half the diameter of the bolt. If you broke that bolt it is highly likely that you will break the extractor. Give it a try first but if it isn’t budging you may need heat.
If it doesn’t come out pick up a propane torch nozzle and a can of propane. You can find it in the plumbing section of your box hardware store. Heat the area surrounding the bolt, close to the bolt but not on it. Do it for a long time before cracking the bolt.
Tools:
Screw Extraxtor set
Hardened drill bits
Drill, if you don’t have one just get a cheap corded drill from HF. Drilling metal eats batteries.
BFH - 5lb mini sledge. Perfect for stuck rotors.
Six point sockets. I only use 12 points if I need double the range of movement that they provide.
I didn’t watch this video but it shows the drilling and the extracting.