ugh, crackling is usually not good and not sure if I ever had one recover after hearing that.
Before switching feed completely you can try adding a little boiled smashed egg and see what happens. It should be easy to digest and nutritious.
To me VETrx is nothing more than vick's vapor rub.
But some people do swear by it.
If I had to guess, I think that crackling is fluid in the lungs. I've had the crackling in very young chicks, but it has NEVER actually been a respiratory illness. Either from them burping and not swallowing/gobbling back down a little juice and aspirating. Once was from me trying to get fluids down and not knowing what I was doing. But they all came from NPIP places and were never exposed to anything besides the hatcher or brooder. But I don't know if they can actually get illness with no exposure to other birds.
Also, I have to note my previous post said 1/4 teaspoon salt/soda for electrolytes and SHOULD say 1/2 teaspoon each.
I would get vitamins on board for your duckling. (if it were me. Even though FDA can't say vitamins help when your under the weather, my personal experience says it does). Never mix electrolytes stronger since those can be overdosed on with things like salt and potassium in them. Vitamins will not OD a bird. Any extra will pass in excrement. I do a straight drop to the beak a couple times a day for those really trying to fail. Has been successful but not always.
Your duckling does not need the whole area to be warm. It just needs a place to warm up. This may be contributing to pasty butt.
I wouldn't worry about pushing feed... but if she isn't moving around, eating and drinking.. I would push fluids everytime I woke up to check and not less than every couple of hours. She won't die of starvation but dehydration is deadly. If they don't drink I drop drip just below the nostrils and they usually gobble it when it rolls around into their mouth.
All of my experience is with chickens so far. Won't be getting duckling until June. So please don't go just according to what I say, but always do what makes sense to YOU. And feel free to get second opinions!
For just one, you might consider using a heating pad to keep her warm so she can cool off if she needs? I don't know how feasible that is for goslings verses chicks. Just beware of the auto shut off. Most people who use that method buy one that has a bypass feature. You might also consider buying the heat lamp with a
red light (reptile dept) instead of using (an electric?) heater.. may actually save you $ in the long run verses your current heater and if it happens to help your girl out, win win.
Hang in there!
She's a Sebastopol goose, and was pretty darn expensive.
That should be nice. 
