I got to the point where MRPI was installed, but
;log mrpi wouldn't run anything

So I used the following advice:
copying the Update_KUALBooklet_hotfix_*_install.bin file to the root of your Kindle when connected to your PC, and then go to Settings > Update Your Kindle, then resume from step 5.
Unfortunately
;log mrpi still won't run anything. But after the previous step, the KUAL option appeared in my Library (after copying Koreader including it), but nothing from it can be run.
Any idea?