The brain is incredibly plastic, especially in a young one. Which means that in the face of brain injury, it is capable of developing new pathways to make up for lost function. But, often with a chick that is so compromised at hatch, it will not thrive as an adult. The difference between doing intensive rehab, which you are doing, and culling immediately, which some other hatchers would choose to do, is... I'm sure you know, the difference between the poultry keepers attitude towards their flock. Are they pets, or are they farm animals? I'm delighted that your black chick is making wonderful progress. I'd not be surprised if that one goes on to become a productive flock member. The yellow one... only time will tell. You're doing a great job. Does she hold her head up well? Does she crook her neck habitually in a particular direction? As I recall, you said she repeatedly falls to the right? If you can get her to participate, you might try blocking her from moving forward, get her to stand, and gently push against her shoulder/mid section from the right. When she is able to stay upright as you remove your finger, you can progress to providing a gentle push from the left side to encourage her to shift more weight over to the right leg. Of course, you have to have a compliant chick to do this maneuver! Your pushes should be very brief, less than a second each, and very gentle... only enough to provide the mildest movement fluctuation. See if she will tolerate up to 3 repetitions at a time before increasing, and only progress to the resistance from the left when she is able to stand fairly well without support. These maneuvers may be too difficult for her. In that case, you would back up and try providing the stimulation at her head, working on strengthening her neck muscles. In human development, head control comes before trunk control. So, you might want to try providing gentle neck therapy. Use her natural chicken behaviors. Can you cup her in your hand, and get her to focus on a particular object. Then move her body with the goal of having her stay focused on that object. ie: her head will not move but you are moving her body up/down, left/right. You'll want to pay particular attention to getting her to flex her neck to the right. Give her plenty of visual stimulation. LOTS of things to peck at. Perhaps hang some colorful beads at different heights in the brooder. Put squiggles and dots on the walls of the brooder with a sharpie.
If you choose to cull, IMO, decapitation would be less traumatic for the chick in the long run than the CO2. Perhaps more difficult for you, but less lengthy stress for the chick.