The folded down hem would help the curling, but I think it would be bulky for this. Have you tried a little blocking? I know it's acrylic, but you may be able to tame it by putting a wet towel on it and letting it dry there while the sweater is flat--may have to pin it.
The grossgrain ribbon--I've had sweaters where there is ribbon on the inside of the edges where the buttons go. Maybe they mean to just sew ribbon to the inside edges.
This stinks after all your hard work. :(
"Trust the pattern!"
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IngridKH on Ravelry