Perhaps Ren'Py just doesn't like OTF, try the TTF version from Google Fonts.
Have fun!
I'll give it a try tomorrow, falling asleep here

*Myriad is otf as well and it's working great btw.
Otherwise, I've finished with transforms, they are now reusable and not repetitive (I think we actually had simple fade transform defined 3 times). I've also removed 30 (not kidding) fonts from the game folder and replaced them with new fonts that living found whereever they were used.
Default font is still Ren'Py and I didn't mess with anything Gismo did, just messes I (we) made when trying out different fonts/graphics. SlaveMarket screen, for example, looks a lot better now I think (Dark should be happy about it, you hated the font we used).
There is still work to be done, we'll prolly leave some old fonts that blend well with the game as well, but it's a matter of checking out what looks best (and I am not the right person for that job, as I've said, I didn't really mind us having 40+ fonts in the game

).