Looking at your photo, I can see that the channel appears to slope downwards where the fixing hole is, but also the horizontal part of the channel after the slope looks like it is bulging outwards. What is stopping the trailer hitch from fitting, the sloping part or the bulging part? Can you take a photo of the part that is supposed to fix to it? One obvious question is, do you have the correct tow hitch for the station wagon, and not for the hatchback? I have fitted towing hitches on both FD and GD hatchbacks, and the underside of the chassis channel is flat.
Where you have a bulge underneath, is that the profile of the metal, or is it a thick layer of underseal that has built up over the steel?