could you please try to save content of your /opt0 and /opt1 folders. (one of them contains "confidential" per device ID, so don't share them in public) Just copy them over to internal memory (/Data).
There is a small partition used for saving settings (also by PRS+). It could explain why your problem persists despite firmware installs (it doesn't get overwritten as far as I remember), but it gets flushed when you reset firmware. Mm....
It struck me as odd as the only successful firmware version I'd put on it is the one through Hack 300 Updater so it should have matched the 'New software version'
That's not a problem, hack updater fakes the version string, to avoid "your firmware is newer" problem.
I think rootfs part should be compatible. (there are people who used this route, they would have problems otherwise). But it was long ago, so maybe they were replacing root, and I just don't remember... :S
here are dumps
for 300 that I have.
18160 is the standard US firmware.
24180 is probably UK, I don't remember it anymore (and didn't leave a comment,silly me)
If this isn't enough, we should ask VictorSG, I recall he had dumps of pretty much everything related to Sony Readers.