Hi Fishy & ev,
What type of fish is that- is it a fantail or one of those that tends to be kind of round anyway?
I think your diagnosis/plan sounds good. Your pics look an awful lot like what one of my fish was going through one early spring - although yours seems a bit more rounded (hence the question about type of fish). It kinda looks egg-bound too, so that could be something to look into.
As it turns out my fishes swelling was definitely caused by feeding during cold weather (when their digestion is slow to non-existent) . I was totally freaked out thinking my fish had dropsy - but a koi pond expert on the forum I was on insisted that it was constipation, that a number of his fish go through this every year and its generally self correcting as the water warms up. It sounds funny but its not, it can be a problem. Feeding peeled peas soaked in garlic water is a good way to loosen him up - now that your water temps are better. After a few days of that I found small hard chunks on the bottom of the tank that he was passing - the same beige color as the fish food! The swelling began to go down after that, but it took a month for him to get back to normal.