The heat plate I use I think could make a burn mark raise the plate up. I had to adjust the one I have quite often they grow so fast.
They do have bald spots as they begin to lose down and feathers begin to start.
They can be stumbly but they could also be needing some Niacin are you giving any niacin supplementation? Ducklings need niacin to have good strong legs even when fed a feed specific for ducklings they can still come down with a niacin deficiency. So it's very important they get niacin everyday especially while growing.
Here is some info
https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/
now that is funny[ Are all duckling so clumsy and fumbly?

They just fall into the water bowl and then get up and go "look, I'm in a pool!" Hahaha]
They do love to climb in any water container they can find.