Most state vets will perform a necropsy for a fee if you want to rule out a disease or find a cause of death. Many of us do home necropsies, which basically for me is just to open the abdomen and examine the organs. When I do that, I would take pictures of what they look like, especially the liver, intestines, and the oviduct. Sometimes we will see lash egg material, basically from pus and egg contents, sometimes yellow fluid when the abdominal wall is opened if the chicken aphas ascites. Opening the gizzard and crop to look for a blockage is good to do. Here is a necropsy video that identifies the organs, and I do a very quick one using scissors and a sharp knife, not as thorough:To try and help anyone else who stumbles across this with the same issue....
I did try the gloved finger ~1.5in or so and felt nothing. Belly seemed normal.
Don't provide extra oyster shells, but we do use a feed with oyster shells added.
Not sure how to even perform any useful post-mortem procedures so I doubt I'd be able to report back with anything other than a mess.
Thank you for trying to lend us a hand. We genuinely appreciate it.