If you've got the box back then try and work out how it was sat in the box, then match this to where the impact points are on the box. If they can theoretically be lined up then it maybe that the severe damage was caused in transit.
The other interesting fact in this case is that the courier twice failed to deliver to the customer, even though "apparently" they had actually visited your property. My guy feel is that they damaged the bike in transit during the first attempted delivery (may have been dropped, or more likely become dislodged from its rack in the lorry with some other items and become trapped. If it's then travelled a reasonable distance then the damage would worsen), the Driver may have then hoped that you will collect from the depot do that he hasn't been the named employee delivering you a damaged bike......
Lots of learnings, but you still have a reasonably good case. If the packaging doesn't line up with the damage I'd be inclined to get an opinion from a reputable LBS to assess if the damage is recent or not, missed on this you may want to pursue the seller.
Jules