I have the same problem (kobo showing bold instead of italics) with Bitstream Vera Sans .ttf. The online font converter created the .otf files as -Roman (not -Regular), -Oblique (instead of -Italics), and -BoldOblique
I renamed them for the kobo, and after powering off and on, it worked! Thanks for the advice
Side note: My original .ttf files work properly on the Cybook Opus.