I would put the little in lukewarm (75F) water deep enough to float in. It's young, so it may not be able to float for long - must be supervised constantly.
After that, I would consider wrapping the leg in an Epsom salt compress. I will do some thinking and get back to you.
My basic notions are - increase circulation, avoid further tissue damage.
And I am not angry with anyone - I know everyone is doing their best. But, wow, that does not look comfortable.
Getting back - any of my research on treating edema (swelling) gets back to - Epsom salt soak, gentle motion, even the right kind of massage, floating in water and swimming.
If you want to try, very gently massage - I mean, just barely touching the leg, no real pressure - from the foot, up the leg. The duckling may hate this, so I would just stick with tub time if possible, and Epsom salt compress. Remember it's a laxative, so don't just put it in the tub.
Thanks - you are so good to duckies.