I'm sorry for your losses.
Looking back at the thread about your chick, it seems that was a tragic accident. So, not an illness.
The EE, you mention she stopped laying eggs a couple of months ago, but did find soft shelled eggs you thought were hers.
There's no way to know why she died unless a necropsy is performed, either through your state lab or you do your own informal investigation.
That said, my speculation would be she may have been egg bound or perhaps she succumbed to a reproductive disorder.
So very hard to know and it can be sad.
Provide your flock with fresh feed, fresh water, oyster shell and grit (crushed granite) free choice. Keep the coop/run relatively tidy.