Things like this happen with Cornish X no matter what the age sometimes.
I probably wouldn't have moved it to the other brooder either. I recommend the chicken hospital be a cardboard box in the house set up with a towel or cloth baby diaper (they work great) for softness and warmth. But that's me. Not sure what the crackly breathing is. Does it eat, drink, poo normally?
In my experience, when a cx doesn't walk or run around normally, they won't make it. I had to cull two 3 week old cornish x chicks that stopped walking around and got listless in my last batch.
Also, small cx are generally not thriving. If electrolytes, probiotics and scrambled egg don't turn them around, they probably won't make it.
I had a runt also in my last batch. She was half the size of the other chicks at 5 weeks old. She ran with them and they seemed to look out for her until one day, she just stopped eating and that was that.
Now I am going to give some MAJOR advice. I LOVE Cornish Cross Please free range them just like your layers. DO NOT feed them 24/7. Let them be chickens not locked up. If you give them space, limit food and let them be chickens, they will thrive and act like chickens! If you don't believe me, visit my thread titled First Run of Cornish Cross and Super Excited! I'm sure you've seen it and hope you've read it. There is an account of my two batches of CX as well as everyone else willing to free range them. We are a bit crazy, but encourage and welcome anyone to join us wanting to treat Cornish Cross as chickens with love and respect just like our laying hens!!