DNSB's instructions should work. But, try them in two steps. Do the first three steps and then disconnect the device. This will boot to the factory firmware. Check it is working and then connect and copy the files for the desired firmware level to the .kobo directory.
And make sure you remove all the files and put all the replacements in. You description makes me think you might not have replaced the "upgrade" directory with the one from 2.5.1. The contents don't change for all firmware versions, but the ones in 3.8.0 is not compatible with 2.5.1.
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