I have a gallon plastic waterer and I add a splash, which amounts to about a tablespoon, before I fill it up. I have about 30 chickens, so this lasts a day, so it gets scrubbed and refilled each night, but they do free range so they have access to and often drink from dirty puddles etc as well....sometimes I really don't know why I bother!!
It's good to hear that the vet recommended breeding for resistance. I believe the official line used to be "cull the whole flock" and sometimes opinions like that are not reviewed and updated and old school thinking prevails.
I currently have 9 bantam cochins (we call them pekins in the UK) that are 2 weeks old. and cute as buttons! This breed is a relatively new addition to my poultry keeping. The parent stock have not been exposed to my flock, as I only bought them a couple of months ago and keep them at a separate location, but the eggs were hatched and are being reared by a broody within the flock that has been exposed/infected, It will be interesting to see if it is exposure from day one that allows them to build resistance(a bit like the vaccine does) or if it is in the breeding. I know that sounds a bit callous, like I'm experimenting with their lives, but after so little problems with last year's chicks showing Marek's symptoms, I'm not sure breeding for resistance can be so effective after just one generation. My main flock now consists of mostly farmyard mutts and that may have something to do with their vigour and resistance, but my money is on low level exposure from the start.
I'm sorry to hear your silkie is not too good at the moment. I'm of the opinion that once the flock has been exposed, it's pointless isolating sick birds and since dander dust gets everywhere, and that is the means of infection, just flapping their wings will result in it being shed into the air and potentially inhaled by others. It can travel on your clothes hair and shoes as well as in the air, so isolation isn't always effective anyway and I think there is an element of some birds will just be more susceptible. Reducing stress and keeping them as happy as possible is the key in my experience.
The two seriously disabled ones that I put together, had the most shocking fight when I put the second one in my little infirmary. I thought the first one was at deaths door (it was her second and more severe bout of Marek's paralysis) and I had more or less given her a date to be euthanized as she was nest bound and soiling herself every day with no improvement for weeks. Once the second one "invaded" her hospital space, she decided she had something to fight for and it was quite shocking to watch two really handicapped birds fighting.. After they established the pecking order and the death's door one amazingly won!! they became good pals and grew stronger together and competed for food which is always a benefit to keep them eating. Once the weather was better and I put them outside for a few hours a day on grass, they really started to show significant improvement.
Try not to beat yourself up or look to blame yourself. You are doing your best for them. Go with your gut feeling about what is best for them. Sadly some will die despite your best efforts.... it is the way of the world.... but rejoice in the ones that rally, as they are the ones that give hope.
I can't say whether your silkie will make it or not but all you can do is give her as good quality of life as you can and be prepared to end it for her if it becomes intolerable.