I have 2 Khaki Campbells and 1 Cayuga. The one in question is one of the Khaki Campbells. It is larger and less active than the other. I have a round feeder with a bunch of access points for them. I am thinking of placing another in the brooder. We just upgraded the brooder to a larger one earlier this week.
KC's are a good, medium sized duck. Big fan! Fortunately, a niacin deficit should be easier to correct with one than a larger breed like Pekin. I think the set-up sounds fine. I have a couple of similar feeders that I've noticed a duckling or two struggle with. One of mine has a top you can pop off for better access, which I think they find preferable, even if there is a bit more spillage.
They are getting free access to chick grit for the last week (starting at 2 weeks old). We didn't start giving treats until they started the grit. But they get maybe 1/2 cup of chopped greens 2x a day (morning & night) and they get small bits of dried mealworms occasionally during the day (mostly as a training treat to ease them into being handled). I try to give more greens than those though as I don't want to overdo the protein.
A good rule of thumb for treats is not to exceed 10% of their daily diet. Possible the ducking is filling up on greens and eating less of it's food than she should.
If she is a girl (like ordered) than maybe she is getting pushed out by the other 2. Since they are almost 4 weeks i'm not sure vent checking will be easy (especially as this one doesn't like being handled).
Honestly, I wouldn't worry about that. Vent sexing can be dangerous if done incorrectly. I wouldn't attempt it unless someone with experience was present tbh. Just trust the professionals at the hatchery did it correctly, and if not, you can just contact them and I'm sure they'll remedy the situation in some way.