We can now mark everything as solved!

2. Indeed, "font-family" worked as expected. Thanks for the look up!
3. I thought "Working directory" referred to the location of MAME's executable, just like in the main options. Then I saw a ".qmc2" folder residing in "../DIST/". Changing "../DIST/" to "./" in the "Arcade mode setup" did the trick.
Thanks a lot for the support!
