when I hold her for long periods of time, she still seems to sneeze though
Can you get some photos of the white boney pieces. I must have missed that part.I also found another hard bone like object in her poop that has also much remained the same being dryish and light brown color, its gotten a bit less dry and sometimes is dark brown and goey but its inconsistent, either way worried about every time I find white boney pieces
It's good that her breathing is improved with the medication. Sneezing if held for a long time is interesting though. Does she sneeze when she's not held?
If the poop is really dry, then you can give her 1 tsp coconut oil or mineral oil twice a day for a couple of days to help with constipation. A dark brown gooey poop sounds like Cecal poop which is normal. This comes from the Ceca and is only produced about 1 out of every 8-10 poops, so it's an intermittent type of poop.
Layer feed does provide calcium, but some hens need more or they don't absorb the calcium like they should. Giving extra Calcium for a few days should not poison her.