Thank you interweb friends, for your sympathy and your suggestions on how to fix this frock.
I bit the bullet and took Lsaspacey's advice (special 'mwah' to you!), and released the front darts, even though this meant undoing the overlocked and hand-stitched hem and pulling off the interlining. Ugh.
But it's worth it to have a wearable dress! I can move! (and eat, and laugh, and sit...)