Neil, points [2] and [3] performed successively won't change anything So, I think smokefree can omit them and proceed to [4]

smokefree, no need to erase fw_all.bin. However, if the flash chip is broken, then erasing/writing any region (including region 14, containing fw_all.bin) can help reveal the chip failure.
Then most probably only hardware repair will help. Though, I don't suggest doing this test right now.