Learning to sew: Mishaps with my muslin. Sigh. What to do?

Here it is, at long last, the muslin of Butterick 9098, circa 1950s. What do you think? There are flaws, alas. The fit needs improvement. But isn't Atchison adorable? Cute? All that? And more. Of course.

The side view.

The rear view.

The bust darts. What is causing that unseemly fold?

The pleats. A disaster. Yes, indeed.

So. Dear readers, extraordinary sewists, you. How do I fix my muslin? The muslin is made of a quilting cotton, courtesy of a pal. The dress will be made of brocade, one of the suggested fabrics.

Yet another problem: The dress falls off the shoulder. Is there a way to fix that problem?

Here is the way the dress should look ? the slim skirt view, of course.

Please. Please. Share your experience, strength and hope. I await your replies.