Sorry your hen isn't well.
DE in the food is completely irrelevant and used by many companies already... approved for use as an anti caking agent up to 2%. But proven ineffective at internal parasite control.
Why worm if the other birds was confirmed to not have worms? Worming an already ill birds can just push them over the edge.
You have an awful lot of supplements going on... but what are you feeding including treats?
When was the last time you saw eggs? How is your weather? How old are your 3 new babies and where did they come from and how long have you had them?
I hope it doesn't go that far... but if you lose another hen I suggest getting a necropsy done to determine cause. Here are links to help achieve that...
https://www.backyardchickens.com/threads/how-to-send-a-bird-for-a-necropsy-pictures.799747/
https://www.aphis.usda.gov/animal_health/nahln/downloads/all_nahln_lab_list.pdf
Alternatively, they are getting that age to start seeing things like internal laying or reproductive issues.
Milk thistle ... I think is supposed to be like an anti oxidant for the liver maybe?? How long did it take from symptoms to passing on your leghorn?
Sorry for your loss also.
Hope you get answers and your hen recovers quickly!