I suggest to go and try using the "idme" command (I guess it's also there on the K2) to set the serial number and board ID of your new motherboard (back it up!) to the data of your old motherboard. I think this is the logical approach after failing to retrieve or change the WAN MAC address.
Additional note: If it's somewhat like the K3, the WAN MAC address is stored in a section at the beginning of the flash memory, near where the serial number and board id are stored. Should be retrievable and changeable by someone who knows her/his hex editor.
As for the question "Now, I wonder why they sell these motherboards if they can't be used?", I guess the answer simply is: "because people buy them" and this way, they can make a bit of money out of the debris left after having destroyed the screen substrate in some stupid accident. I guess these boards won't ever work as a replacement out-of-the-box.