28. what's the reasoning behind melee powers (like cleave) doing damage on a miss?
Because hit points have never really (or realistically) been "meat". People have tried to shoehorn them to be so, but it has never really worked since they started trying around 1975 or so.
Hit points are instead a pacing mechanism to reflect a "Hollywood/movie reality" where someone fights on, unscathed and undaunted, until they run out of "mojo". At which point, the next blow kills them (or, at least, disables them) cleanly. Think how many limbless, eyeless characters you see in the average RPG. Usually next to nil. A system to represent "meat" would have independent wounds (multiple gashes to different parts of the body won't suddenly cause you to keel over dead) that had the capacity to leave lasting disabilities and death would come through infection, shock and bloodloss far more often than a "killing blow"*.
So - hit points tell us when a creature (character or monster) runs out of "juice". They are no longer able to sustain the sheer will needed to throw themselves out of the way of that potentially fatal blow. Their concentration on that next parry is faltering.
So, it makes perfect sense that an utterly devastating attack might drain some of that chutzpah, mojo, vitality or whatever you want to call it - even if it is not executed perfectly. In short -
all attacks before the killing/disabling blow miss, but some miss more convincingly than others.
29. can someone help me wrap my head around level one characters adding their constitution score to their hit points, in terms of both fluff and crunch?
Those with high Con have more vitality and will-to-life than those without, hence they have more hit points.
From a game-balance perspective, this gives Con something valuable to do - makes it a useful stat to have and gives important advantages to those classes for which Con is a primary requirement.
*: For a game system that actually does this, see HârnMaster.