I'm right there with you. I'm due at the end of March and I'm standing in my brother's wedding May 30th. I need a gown that will will work for nursing, pooch, and a sensitive C-S scar

(I also need it to be knee-length and black).
This may be dressier/spendier than you're looking for, but I was looking at something like
this I figure that the high waist and ruffles will mask the tummy and the top will drop easily for nursing (and I'll use a sling/shawl for discretion.) Of course I'm not sure how fitted the lining is so I'd need to see that too. Anyway, just another option. I think that something like
this might be workable too with the flared bottom and the buttons for nursing access plus good coverage for a nursing bra.
ETA: I had to add that Nordstrom has NO time limit on returns, so if you purchase a dress there that ends up being a no go when you put in on 2 mos. post partum or you're not sure what size to get, etc. You can return it for a refund of the purchase price any time.