Thank you all for your help and advice.
We decided to put her down last night after all. I gave it a good try and worked on helping her for 9 days with no sign of improvement. It was distressing to see her getting into such awkard and uncomfortable positions every day and not being able to untangle herself on her own. Almost every time she decided to move around, she ended up getting her right foot/claws stuck on top of her right wing and inadvertently holding her wing down (looked very uncomfortable and maybe even painful). Sometimes she'd end up on top of her water dish (with no way to drink) and have to wait till someone discovered her and could move her back into position. It was a lot of work trying to care for an indoor chicken with paralyzed legs - I had to change her paper towel diapers, adjust her position, and give her vitamin drops and acidophilis about 6-8 times a day. I have a busy schedule with my job, kids, dogs, school, and my other chickens and I just couldn't see doing this for 1-2 more months, not knowing if recovery was even possible.
If I still thought it was botulism and there was a chance she would get better, I would have kept going, but with the idea that it is most likely Marek's and there is a much smaller chance of recovery (most sites say there is no treatment) I just wanted to let her go. CoolChickens - I'm so glad the hypericum worked for you and appreciate you giving me the info --- I just don't think I could do the hypericum twice a day in her beak for 2 months, not knowing if it'd work (she's a fighter and it was awful trying to open her beak enough to give her those flushes that first day). I was also a litle nervous about putting a possibly contagious chicken back into the flock (what if it's not Marek's, but something else?).
Chickens vaccinated for Marek's can still have Marek's disease and be carriers - they are just less likely to come down with symptoms of Marek's than unvaccinated chickens. I was wondering if my two new hens were carriers of a different strain of Marek's than my original flock, but then remembered that my last 2 batches of chicks I raised in Feb and April are totally fine - no lameness or health problems, so that elimates that possibility.
Many of my chickens came from the feed store as chicks and were not vaccinated - 2 of my groups of chicks were raised by mama hens, so it wouldn't have done any good to vaccinate them either - the vaccination doesn't work, unless given to chicks before they are exposed and these chicks have to be kept away from other chickens for 10-14 days, giving the vaccine time to work. The sad thing is; this particular chick could have been vaccinated - I just didn't choose to pay the extra $30 (for 30 chicks) because I didn't think it was necessary - hadn't had any previous problems.
Maybe my little Welsummer just became extra susceptible to Marek's because of her body being stressed when I changed her feed to a type she didn't like and wouldn't eat? I've heard that Marek's disease attacks chickens when they're stressed and since she wasn't eating her food for 2 days, maybe that stressed her body enough for the disease to take hold of her? I hope the rest of the chicks stay healthy and don't come down with any symptoms - so far, they're doing great. I believe my older chickens are already past the danger zone.