You might bring this up in your essay, but how different would it be (both in-universe and out) if Sansa was, say, 15 or 16 when she gets married to Tyrion rather than 12?

Well, that gets a bit tricky. The whole “Joffrey is your father” argument might fall apart if Sansa’s reached her majority. 

Would make a tiny bit less creepy, but there’s a lot of creepy to go.