On my semi-bricked one I poked around,
My main core is halted.
But the flpr core is running.
I tried various combinations of halts and erases but never managed to turn off the memory protection that prevented me from properly wiping the chip back to defaults.
Try adding
CONFIG_FPROTECT=n
To your prj. Conf?
It’s something I just found on the Nordic forums on a post about similarly bricked DK versions of the board