Re: Not necessarily blunt object, perhaps a curved panel
I think the hard impactor came from the solid rocket booster. Such an impactor could be a curved panel, with mostly sharp edges, maybe even pointed corners. If this is true it would be spinning rapidly after the impact, and there would indeed be a deep gouge on the underside of the wing.
Columbia Debris Hit
The people at Thiokol would know. They were responsible for the Challenger launch failure. Maybe they were hoping to dodge a bullet this time too.