No advice on the blazer, but have you washed it yet? I have one from Forever 21 which was about 3" too long when I bought it, but after washing is the perfect length. Oddly, it didn't shrink anywhere else, and hasn't changed sizes in any washings since!
I wear my flowy wide-leg jumpsuit with a waist-length fake fur jacket, but reckon any short jacket that ends around the same point would work. You want it to balance the wide silhouette on the bottom half.
Having said that, am going to nick the duster coat idea when the weather warms up!
Slightly too long better than too short on a wide leg style I always think - you can wear higher heels that way.