Grand Marnier would be great sub for the extract. Might wanna make sure you have extra so you can have an extra sip or two because it always smells so heavenly when you measure it out for something else ...
I don't think of graham crackers as being more or less sweet than 'nilla wafers, just different flavors. /shrug But I'm all for substituting something you're not sure about for something you know you'll like.
I'd do the first half of the recipe (without the topping) and then freeze it. Once it was back at room temp I'd do the topping part.
Let us know how the changes come out.