I love mussels, am surprised how many dissenters are on this thread (mussels are a popular restaurant menu item).
Anyway, my specific thoughts are:
Apps- since the entree is “light,” you could go heavier with an app. I’d keep the cheese/nuts and probably sub for the second either mini crab cakes or mini meatballs or stuffed mushrooms (I like a hot app with a cold/room temp one).
Main- I do love frites with my mussels but so long as the bread is good, I wouldn’t say is necessary. I do think you need something besides mussels and green salad, though. I like the roasted veggie idea, or frites or roasted potatoes (maybe patatas bravas?).
Dessert - I love your dessert and I’d also add some dark chocolates bc some people just “need” chocolate after a meal
