If the grit and oil don't help, and I'm guessing they wont, it may be time to euthanize. It sounds to me like she has ascites. The fluid in the abdomen can restrict the lungs, and make the breathing more labored.
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Your update sounds encouraging and made me smile and giggle. Behavior is your best indication of the state of well being of a chicken. Keep doing what you're doing, and keep a close check of her crop. Check at night and first thing in the morning before you feed her.
If the crop is still full in the morning, it means her crop issues are ongoing and need to be addressed.
I hope your hen does well. I just wanted to comment on her being 7 and still laying ! Wow, I had no clue..![]()
She may have a condition called pendulous crop. http://www.the-chicken-chick.com/2016/02/pendulous-crop-in-backyard-chickens.html It prevents the crop from completely emptying, setting the hen up for chronic crop problems. It can be helped by having her wear a crop bra. Take a look at the article and see if any of what you learn applies to your hen.