I have a 5'x5' pen that mine go into during the day if they can't manage to free range or are getting picked on. I scatter feed on the grass in and around the pen so that the other birds come and feed with them. This means that they feel like they have competition for food which is always good for encouraging them to eat and have to move around which is good therapy but in a safe environment where they are not getting harassed by the others. Plenty of fresh air, grass and sunshine when possible and on an evening I encourage them to walk back to the hen house after the others have gone to bed. I find this helps to build their confidence back up and allows them to learn to manage their disability. Sometimes it seems a little cruel to make them walk when they struggle but I find that once it becomes a daily routine they get better at it. I don't think they suffer pain with the lameness with Marek's, just frustration that things don't work properly and panic when they fall over and can't get back up. Once they stop panicking, they usually find ways to manoeuvre themselves back into an upright position. Of course there are birds that don't make it and deteriorate to the point that they have to be euthanized but giving them their own protected place within the flock and keeping them moving and encouraging them to eat by feeding them whatever they will eat (soaked mixed corn, scrambled egg, meal worms, soaked bread) rather than trying to ensure a balanced diet of grower pellets or pump medication into them that they don't want and get sick of, seems to be working for me.
So in regard to your question about can they recover from leg paralysis..... Yes. I've had some make a full recovery in a matter of a few days and others take months of TLC to make a partial recovery which was enough for them to return to free ranging with the flock and even laying. Secondary attacks weeks, months or even years later can happen and are usually more severe, but quality of life in the interim is usually good once they have regained their confidence. I find isolating them usually causes them to give up hope after a few days/weeks and deteriorate no matter how much TLC you lavish on them.
I have read articles recently regarding problems with the Marek's vaccine being an "imperfect vaccine" that may actually be causing more virulent strains to become prevalent. I don't vaccinate my birds and I allow my broody hens to rear chicks within the flock. I have a relatively mild strain which seems to be the norm here in the UK and I lose 2 or 3 birds to it each year, but I hatched about 30 chicks last year and 50 this year, so in the scheme of things, that is not a big loss although I feel each individual death in my heart.
Good luck with your remaining birds. Keeping them happy and stress free is the key to fighting the virus.