This is a great story. I love the way that you have redeemed Goyle and given him a career and some self-awareness. You've articulated a fairly inarticulate character and kept him recognisably the same person as he was in canon. I've always felt sorry for poor Marietta. Hermione got a bit too wrapped up in how clever she was in my opinion and failed to think things through and empathise (as teens who are wrapped up in what they are doing often do). It's great to see the consequences of that many years later for Marietta.
These two are great together. It's such a relief at the end when they fall together at last. It's so clear that it's the best thing for both of them.