$readini() has nothing to do with the issue. In your command, it's not even evaluated, since you used a single slash. In fact, if you use // it works as expected, ie like /font -z $null $null, ie /font -z.
The problem is when you pass mirc an invalid font name, in which case it chooses to change the font to Arial 12. It seems to be a bug; I would expect mirc to ignore invalid fonts and give the same result as /font -z (with no params). Another clue that this may indeed be a bug is that, after specifying an invalid font, $window().fontsize returns 12 but $window().font returns $null.
