[Mark Nord]

So there is progress....

Flash-Script will do the following
Copy new_opt.300.img to /tmp/ - a not exported non user visible part of the readers internal file-system
compare both by checking the md5 checksum
actually rewrite "flash" the firmware

So a MD5 failure may point to corruption of new_opt.300.img while copied form /Data/ (the readers exported part of IM) to /tmp/
or a incompatible MD5 command.

But to just understand your progress:
You managed to run the Hack 300 Update file - and actually installed a 505 fw ?
And now you can't install the original one, right?
Using which FW-image? The one from the PRS+ project or a porkupan/boroda version from the-ebook.org ?

If you got the 505 fw up, it should boot through and offer a main-menu. Though eating up battery, due to incompatible power-management.

If the reader is still stuck on "opening book" page, there is a massive problem with the readers-filesystem or the database.

Have you already done a backup of your content? And re-formated / chkdsk the drive named READER?