Given the severity of the bend on the fork, you aren't looking at a "dent" in the downtube but a buckling of the tube itself. Nothing 'hit' the tube to dent it but, rather, the tube tried to fold upon impact and is well on its way to failure. Looking at your first picture, the top tube looks like it might have some curve to it which the Volpe didn't come with. Forks are beefy...beefier than the frame... and if you can manage to bend them like you have, you've likely damaged the frame.

As for the thought that this is in a low stress area, this picture borrowed from here



would say otherwise. The area where you buckled the frame is in the pink zone of the downtube and is one of the more highly stressed areas of a bicycle frame. Don't try to straighten the tube, drill out a hole and punch out the dent, use a blow torch, think of it like factory crimping of a low stress area nor any of the other suggestions offered here. The frame is broken. It's not safe to ride and it is now scrap. Treat it as such.