If the car doesn’t start with the modified file, will there be the possibility to flash back the last bin file I read with Nefmoto software without bench flashing?
No. Don't flash unless you have a bench harness to recover in boot mode.
Does 1024kb’s BIN file means that the flash memory chip has been read 100% (all the IMMO code info etc.) and could be used for BDM or OBD bench flashing to other 551K ECU if something goes completely wrong?
No.