I can't opening your project because I'm using VS 2010 and it's incompatible.
In my .Xaml, I'm using neither styles targeting the FontFamily nor apply a 'Font' properties (like color, size, etc...). I'm using the default values.
I tried your code :
// I added your FontFamily setter.
-> The font is applied but the black special character with '?' is always presents.
I noticied that when I edited cell, the accent is applied (attached file).
=> When data is loaded, accent isn't here.
=> When data is added by editing, accent is presents.
Thank you for your answer.