I can only suggest trying harder to enter the pre-boot mode.
Loading ldr.bin would help only if the fw_all.bin boots far enough to load it. But it seems that the device crashes while running the bootloader (which provides the pre-boot mode).
If running ldr.bin or entering the pre-boot mode is impossible then I don't know how to cure the unit using purely software methods.
Then in most probably can be cured by unsoldering the flash chip and flashing it with a healthy dump.