The thing with Icelandic is they don't have a standard, so they are a wide variety of colours, crested and not crested, and have a variety of comb types....they could be Icelandics🤷🏽♀️
With ducks the females quack and the males have a raspy whisper, as ducklings they sound the same and they grow the same. If you have one not growing its failing the thrive for some reason.
Ohhhh my...I can tell you what I personally do but I'm not telling you what to do. In my experience crossed beaks get worse as the chicks grow, much worse. A lot of the time they struggle to eat and drink which can lead them to failure to thrive. They usually end up starving. Myself I cull them...
The same as the females. I find with mine the males necks are a little thicker and there is a bit of a size difference when they're standing side by side. Other than that I just wait to see who lays an egg or who mounts who then catch them and put a band on their leg so I know who's who