hm, you should be able to fix it with only with the "Load MMC0 over USB storage" option (if it is fixable and not a hardware defect).
That said, I'm not sure how the filesystems are laid out on the KDX b004. I guess from the sizes that sdx1 is rootfs, sdx2 is /var/... and the third one is the user store.
From the logs you have posted, the initramfs can not mount the root file system. So don't concentrate on the rest of the partitions yet, but rather check the root file system. Can you mount it on the host machine? Before messing with it, I would suggest to make a full dump of everything, i.e. the full block device (not just single partitions). And try to run e2fsck on the root file system then.