in my experience you just can't get them tight enough with a 6mm allen key...tis fine for the bulk of the tightening, but you really should get a spanner on chances are they were loose then..
mine does it too...apart from having to make sure my headset is actually tight, it really doesn't make much difference to me. Probably a bit different if you actually go places quickly over bumpy stuff though?
it's designed like that.. hmmm...doesn't seem so easy! it still needs to be flush inside where it sits against the steerer...so i can see another crack probably starting again from the steerer side where you might not catch it so easily...tis a strange place for a crack though. good shout. Uneven tightening may have caused it...perhaps even riding with one bolt loose(ish) meant that the stem could move slightly and thats some sort of fretting crack? unlucky i'd say. adam