Hi
I'm sorry you lost the older girl. Not laying for a year suggests she may have been laying internally and that bloats their abdomen and eventually leads to death.... she may even have ruptured if there was blood.
Video or at least photos would help (as requested previously).... it is very difficult to diagnose over the internet anyway, but doing it blind is at least twice as hard. Also, I don't think you answered the question about additions to your flock in the past few months and if so, where from.
It would also be helpful to know what you feed them including treats.
I am leaning towards Marek's since you mention that she is walking on her hock on one side. Marek's can cause a multitude of symptoms and loss of control of a limb is one of the commonest. Usually the bird will stumble or appear drunk with a neurological impairment whereas, when the lameness is down to pain of an injury, they will consciously favour the other leg to prevent putting weight on the bad one. This is why a video helps in these circumstances. The older one falling on her side does also suggest it may have been Marek's with her too. Marek's also causes muscle wastage and in the later stages tumours form inside them. It also suppresses the immune system and makes them prone to infection. The vent/poop smelling bad is usually an indication of an infection or necrosis.
Mixing birds of different ages from different sources greatly increases the chances of introducing Marek's into your flock.
If it is Marek's, the best you can do is offer a good quality supplement like Poultry Cell, Rooster Booster or Nutri Drench, and perhaps a probiotic or fermented feed to support the immune and digestive systems. I also find that giving them access to sunshine and grass beneficial with Marek's birds.
Sadly the easiest way to diagnose Marek's is via a necropsy when they die. It is contagious but because of the long incubation period for the virus (minimum 3 weeks but can be months), your flock has already been exposed so little point in quarantining sick birds with it in my opinion. They may benefit from being kept in a safe environment like a cage with their own food and water within sight of the flock though as that helps to encourage them to eat and once they stop eating, they don't last long.