A.
I had to be a bridesmaid when my son was 6 months old and was worried about how that would work - strapless longline bra, spaghetti-strap dress, and all. I had a bottle for him, and I think I only nursed him twice during the reception. I had to leave and go to my room to do that, though. He was already starting to eat Cheerios and baby foods at that point, and wasn't nursing as often.
I'd look into a skirt and top combo - perhaps with the top in a stretchy/non-wrinkle fabric. A jacket over the top will give you extra cover.
I've never had luck finding a decent nursing dress that didn't look dowdy, or very obvious (HI I'M NURSING AND HERE ARE THE FLAPS) or both.