Load V6 RC12 into the router (drop the files in) and then see if you can flash the board firmware.
That way you can upgrade the board firmware, and then when you restart it should be able to start in RC 12? I'm just guestimating this...
I upgraded the routerboard firmware to 2.29 from 2.28; however, V6.0rc12 still didn't install and the system isn't finding any more despite others claiming to have upgraded their RB450G to firmware 3.02. My license allows upgrading to V7.x.
So, why 6.0rc12 isn't installing?