Hens don't sit only on their own eggs. They don't even know whose eggs are whose, which is why you can try to entice them with fake eggs, chicken eggs, etc. Runners aren't generally the best mothers, so your Runner hen might never go broody. No worries, though, as long as you have at least one...
Those both look like hens to me except that I've never seen a Khaki hen with a black wing bar. That might just be a breeding variation.
Do you say the Runner hen isn't laying yet because you keep the Runners separate from the others? The reason I ask is that, if you don't, I have a feeling...