We have had the suspect hens/rooster for around 4 weeks. But, we got ALL of the birds from the previous owners (all the birds they owned). They were completely penned up in a chain link coop with no way of getting out or other roosters (none in the area anyway - city) getting in. We thought there had to be a wild rooster around, but have not been able to find one. The suspect we have does have darker feathers and slightly straighter/less rounded at the neck. The tail is pretty similar to the other hens - only VERY MINOR differences, including a little more pointy. The only reason we suspect this one is that we saw some on-line pics of a buff oprington rooster that looked almost identical to ours. We have also noticed that he/she will not go in the nest boxes. We isolated him/her for the last couple of days - no eggs, and slightly noisy as we get close, a little more aggressive when we try to let it out.
As for the egg left in the coop, I am as sure as I can be. There is no way it was there for 10 days. We have a hen that likes to eat eggs. We collect several times a day and we clean it out completely every evening - empty the boxes to the wood frame, and wipe down. 100% cleaned every evening because of the one that eats them. (We have been having better luck in this area lately as we have added a couple golf balls to discourage and collect more often. She seems to be losing interest. Hope it lasts!)
Based on the pics you posted, I would say ours looks more like the rooster than the hen, but not much more. We counted more fertilized eggs this morning when cooking breakfast. Almost half. I am 100% sure there is a rooster somewhere. BTW, this is the only bird we have that has spur buds of any kind.