Strange mallard behaviour.

I've just arrived at an air BnB for the week and have spotted some interesting behaviour from a trio of mallard ducks who are residents of the pond in the garden.

The three birds frequent the house, are fed by the guests and owners, and were hovering at the back door when we arrived so I scattered some food for them. The male and female A (presumed to be his mate) came over immediately but the third duck, Female B (who looks significantly scruffier than female A) didn't come over to the food. I noticed the pair of ducks were chasing her off and so I scatter a new pile for Female B who then fed at a distance. 

Then the pair went to the pond and Female B who seemed to be trying to sneak into the pond was repeatedly chased out. The duck house in the middle appeared to have a nest and 1 egg or egg shell was visible from the bank but I couldn't see any further in. 

We then noticed that female B had something in a water bowl by the back door. On inspection it was a broken duck egg which was clearly a partially developed embryo but was rotten (the smell was unmistakable). 

I was aware that mallards were monogamous and that there is strong competition between males for mates but I can't find anything about female competition in the literature. Do the females compete for males? Why doesn't female B go to find a new nesting site or a new male? Has female A outcompeted female B for the nesting site And would she then kill or remove any eggs that she found? 

Any ideas, thoughts or research would be very interesting to discuss. I can't seem to find evidence of this online and haven't seen this sort of competition before.