Limited experience but the original Krag screws I have seen are not that hard. I see many on this forum with buggered heads from cheap screw drivers, indicating they were not very hard.
Some of the replacement screws I made for mine were from W1 tool steel, then hardened and tempered till I got nice looking to a color blind person oxide colors. The shop made screws were many times harder than original. On my sporter build the trigger screws were run in and out MANY times with no damage to the slots.
Part of your problem maybe the high speed drills. A micron thin hard coating does not help in drilling hard material the underlying material will get deformed and the drill becomes instantly dull. Machine shop grade carbide cutting tools are needed for real hard stuff.
Looking at the end of the broken screw I did not see the fine grain structure that usually goes with a very hard screw. Try a known sharp file on it. If it skids across without cutting, it is really hard.