+1 for coconut milk and the above, + lemongrass. As you've listed the ingredients, no cream please. The main thing for me is that there is a healthy bit of unctousness to the sauce. It makes me sad when it's too thin. A bit of tomato works well with what you've got, too.
Non! Quelle horror!!. The only change I'd consider is frizzling some jambon or pancetta with the leeks... But I'm a purist anyhoo - I'd make a creamy mussel stew and call it that, if I was adding cream. Love mussels.