Sorry, I've been away for a month so have not been able to contribute.
After just changing the font names, then unbricking, and copying the internal font data on each file using font creator (both in Windows and Macintosh areas) I was finally able to change the internal fonts to Calibri, Cambria and Consolas. I simply copied the data over from the existing fonts - identical to post #25.
Note: the font file name for courier (for example) is tt0419m_ttf, then using Font Creator you copy the internal naming conventions exactly as appear in the default fonts (font family: Courier10 BT; font subfamily: Roman; full font name: Courier10 BT Roman).
The trick in the end, for me, was that for the fonts to install correctly I had to erase all my books first and start with a clean unit with an empty storage area. Then I flashed it.
You may want to try that.
It's been working
great for me and over the past month while traveling I read over 6000 pages in very nice, easy to read fonts.