People have soldered other items in close proximity to Arduinos so just because it had headers doesn't mean its immune to the issue.
I can define over exertion pretty well with a picture of a board that arrived quite recently.
Also people have been known to put too much pressure on the USB connectors and break them off boards or tear the tracks etc.
Others have bent pins or wire leaving parts in the headers that are a PITA to get out.
As far as hardware testing you said you followed all the advice you found so if its not seen by multiple computers with multiple cables and trying the double tap reset trick then there is little else you can try.
If you have access to a well equipped lab and the skills and expertise to chase a schematic around you could go further but that is way beyond the viability of the board.
