I really appreciate all of the feedback.
As far as the size goes, I fear it will pull around the bust if I size down any more. The shoulders and sleeves would be better if sized down, I agree. I'm going to be near an H&M again in a few days, I can try on a smaller size for comparison.
I tried it belted today and liked it (sorry, no pictures). As another option, and because belting the waist is a new thing but I've yet to find any garments that work on me belted (sweaters look awful, too bulky for my short waist).
The sleeves are tricky. I could see about an actual alteration. Or, I could fold the cuffs up and tack them. But it does change the look of the dress. I think the sleeves are meant to be long, as that is the style now, but they are just that extra bit too long on me. As is usually the case.