I am sorry you lost your hen. It is hard to watch.
As far as Marek's, I wouldn't rule it out based on her age. I have one that is declining right now and I am fairly certain that is what she has based on flock history. She is 2 years old. I lost others that were "too old" as well, and the lab confirmed Marek's in my case. I am not saying that is what she had, and botulism does seem to fit her symptoms, but just be aware that age is not a good indicator. Also I read on BYC that botulism in chickens is actually very uncommon. Again, I am not making any guesses as to what she had, I am just sharing my own experience. None of mine have had the leg paralysis either.
I have about 30 birds in my flock, some that are as old as my hen that is sick. I started off with 9 chickens and of that 9, 3 of them died from Marek's, one is now sick, and 3 are perfectly fine. It is an insidious disease that manifests itself in many different ways. It is a hard one to deal with. I hope that she did not have Marek's as it is supposedly highly contagious.
I hope the rest of your birds stay happy and healthy.