At which point, could you unilaterally hard-fork it, e.g. to fix this problem with the difficulty-adjusting rules? Though if that is an option, I would guess i

At which point, could you unilaterally hard-fork it, e.g. to fix this problem with the difficulty-adjusting rules? Though if that is an option, I would guess it would be more likely for a coalition of miners to choose to do so before any individual achieves 50%.