The way that she flails around looks a bit like wry neck. Maybe she has spraddle leg but also wry neck. There are videos of silkies on youtube with wry neck.
I had a spraddle leg chick, I layed her on her back in a tea towel, in my lap lengthwise along my legs so she could not move. Then I did the bandaid thing. It basically keeps the legs from going out to the side. I allowed her to practice standing with me steadying her and after a few hrs she could get up and stand on her own. She is a nice little hen now. The chicken with wry neck was months of hand feeding vitamins, us seeing improvement and then relapses and in the end we had to put her down. My husband chopped her head off.
As far as the gas chamber, I have done it. I put vinegar in a bucket and put a tiny chick that was dying into a tupperware dish. There was enough vinegar that the plastic dish floated with the vinegar in it. I added baking soda to the vinegar and then put plastic over the bucket. 5 min later, I returned and the chick was dead. This method, I had read at the time, is supposed to be carbon dioxide and the chick can just go to sleep. Kind of like people who have carbon dioxide in their homes, they don't even realize it, but it can kill them. We dont do the gas chamber anymore. There is no way to know for sure how it feels.
Last yr, we sent some roosters to freezer camp and seeing this has made removing the head a whole lot easier. I only watch, my husband does it. But it is just so fast and certain. Pretty much immediately, the chicken is dead.
Anyways, I hope things work out for you. It is a very difficult place you are in, especially for a chicken that you have grown so attached to.