... So I think the way is to write the appropiate file directly on the flash using the u_boot command line.
exactly that was my idea...
obviously the present loader wants to boot an uImage by using the ch up button, so in the result the update procedure doesnt work as expected.
anyfile.bin... i just wanted to know if you are able to load any file (e.g. an uboot.bin) from usb stick to RAM. if this works then you can go on and flash the loaded binary directly from RAM to the NAND and would be ready with the recover.
for example: if your backup of the mtd0 is named mtd0.bin then copy the file to an usb stick, fat32 formated, and try to load that file by the following commands

usb reset
fatload usb 0:1 80000000 mtd0.bin

and post the output here (output has to be something like '262144 bytes loaded..'

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oh, and just to prepare the next steps please post the output of

help nand

and the output of

nand info