That is exactly my issue with what I read, yes. He shouldn't have taken any damage. Artistic freedom (aka mangaka kinks) is one thing, but the death is clearly not merely that.
Nah, it said "perfect reflection".
All just a speculation, but you're free to disagree.
My WAG is that once inside his body, the different sphincteres formed a sealed environment. From there it was the over pressure that caused him body's containment to fail, and explode into bloody chunks. Even if Mr Cubicle warrior didn't turn into the world's fuglest firework, the internal damage as the explosion leaked out his orifices would have likely caused a lethal ammount of damage. It would have just been a lingering one, rather then going out with a bang.
on a side note, there would be limitations to the skill. Otherwise it would have had a higher grade. True it was powerful, but just the fact the Bomber could grapple him proved it also had major flaws.
It reflects damage, not attacks. If it reflected attacks mc would be dead by now. And it does not make him extra bouncy either. It is simply that the damage he would take would get reflected back onto the wall.
Remember, when he is flying at a wall, he is actually at rest and the wall (and planet) is flying towards him. If deceleration would damage him, then it instead accelerates the wall/planet (maybe subtracting safe amounts of deceleration at every tick, so planet doesnt break).
Think a lot of that would only work with external threats. Once the damage originated from internal, enclosed spaces, the skill would no longer have surfaces upon which to mitigated the damage. Hence, BOOM, goes the dynamite.