Looks like the rear cover should still be able to come off without being hindered by the broken portion of the bolt.
If there's enough of the bolt exposed after the cover is off I would try cutting a groove in the top, spraying with some 'Blaster, after letting it soak for a bit try to twist it out.
same thing happened to me removing my rear cover. I removed all the other bolts after spraying them with AeroKroil and sort of gently used a rubber mallet to get the cover to move a bit. PB Blaster works well, AeroKroil works much better if you can find it.
Yeah those are off, I can budge it a bit but it looks like the gasket is really stuck towards the top of the rear cover. I'm pretty sure that I should be able to take it off regardless of the damaged bolt.
OK, either side of the coolant pump there are tubes that act as coolant passages. This is likely where you are hung up. The tubes are steel and can corrode with the alloy. Spray the best penetrant you have into the gap you've opened in this area and be patient. Gentle prying next to these points will help but don't get too forceful.