My gut feeling is that it is Marek's I'm afraid. The neurological symptoms can vary enormously with this disease and affect individual muscles/tendons in very minor ways. For instance Marek's can cause them to have difficulty keeping one eyelid open. They often can open it if they really need to but it fails to remain open as it should without conscious effort. The bent toes again, can be caused by contortion from muscle spasm just like wry neck or wry tail which are more common symptoms of Marek's paralysis. Yawning or open mouth gaping is often an indication of respiratory issues which are common with Marek's, so the combination of neurological and possible respiratory along with the juvenile age of the bird and the sudden onset all point heavily towards Marek's. You might want to check the eyes for any ocular signs although I've not had any ocular symptoms in any of my Marek's birds, it can help to lend weight to the diagnosis if you see them.
I've tried a few of the suggested treatments like St John's Wort and turmeric and black pepper, but I find that they sicken of their food being doctored after a while and keeping them happy and stress free and eating enthusiastically is key if they are to recover from an outbreak in my experience, so all I do is give them a palatable broad spectrum supplement (here in the UK we have Nutri Drops which I don't think it is identical to Nutri Drench in the USA but similar.... I'm not sure about the other US options like Poultry Cell or Rooster Booster, but imagine they will be much the same). This helps to support the immune system to fight the disease and probiotics or fermented feed to support the digestive system. Scrambled egg a couple of times a week and access to grass if at all possible in a safe environment like a large cage.
As you are aware, if it is Marek's the others were most likely exposed to the virus at the same time as this bird. It then has an apparent dormant phase of at least 3 weeks before an outbreak can occur but it can be longer than 3 weeks. Outbreaks are triggered by stress, and what triggers one bird may not trigger another so not all birds will suffer an outbreak at the same time. During an outbreak they are actively shedding the virus though so some people feel that isolation or culling is relevant to prevent further or more dense exposure to the virus. I personally keep them as much part of the flock as is possible because isolation often stresses them and stress means they are less likely to beat an outbreak. The ones I have had that recovered from outbreaks were kept on grass in the sunshine whenever possible within sight of the flock.
Once they lose interest in food, I euthanize them because in my experience, they have lost the battle with the disease at that stage and it is only a question of time after that. If they are still bright eyed and keen for food than there is hope they can survive that outbreak and I have had some incredibly dramatic recoveries from severe disability from it, but they will always carry the disease and be prone to further outbreaks in the future, which could be weeks, months or even years hence.
Only you as the flock owner/manager can make the decision as to the appropriate course of action which is best for your birds either as individuals or the group as a whole.
Do be aware that the flock could suddenly turn on your bird that is showing symptoms.
