on worming, i do wazine first, its easier on them, then ten days later i use safeguard- here is a good thread on it-
https://www.backyardchickens.com/a/wormy-tidbits-of-information
lots of folks stress organics for worms, i tend to lean the other way, when you can be sure to take care of the...
one of the hardest things with chickens is there are times when you will be clueless- sounds like you are doing things right- wonder if she got bit by a spider or something
i am sorry you lost her- birds are very good at hiding illness, keep a close watch on the others for anything out of the ordinary, eating, poop etc. What are you feeding? Housng conditions etc would help try to figure it out