I strongly suspect 3 is correct - removing material from the corners might weaken the structure.

I would use a CNC machine to round them more precisely and uniformly though.

You probably don't know much about filing, but there is no reason it would be any less accurate than CNC if done properly.

Also doubt this would affect strength, given the apparent thickness behind it, really depends on the interior, which is almost certainly also rounded for strength, and to prevent a week point for cracks to start.

Haha I was thinking the same when I read he hand filed it

Filing vs. CNC doesn't make one bit of difference.

As it is with any box, there is only so much material you can remove from the corners before it disintegrates into disconnected surfaces.