I believe that you are right in thinking that the preceding health problems (respiratory, cocci and maybe even worms) were as a result of a compromised immune system and it does sound like Marek's but I'm not sure what makes you think it's a particularly aggressive strain. I have what I consider to be a mild strain and when it first broke out I had 3 develop lameness overnight within a week of each other. One deteriorated over the following week and needed to be culled and the other two recovered within a few days, from having been floundering on the ground unable to get up, to walking with barely a limp and eventually being unable to tell there had been a problem. Both had secondary and more serious attacks.... one a few months later that I managed to nurse her through and the other, a year later, that unfortunately proved fatal, but quality of life in between times was very good and the pullet went on to lay eggs and free range with the flock. Another one had the same problem as your pullet, in that she was standing on and tripping over her own foot. She learned to curl the foot and pull it up out of the way and hop everywhere (it took several weeks of tripping and me trying to make a brace and support her before she found her own solution. She was a very petite bird (another indication of Marek's) but it proved helpful since her one good leg wasn't taking too much weight. She was amazingly agile despite her disability and she also free ranged with the flock and laid eggs and got up onto the highest roost at night,, but sadly was victim to a fox last summer.
This summer I had 28 chicks broody reared within the same flock... I don't isolate Marek's birds since my flock have all been exposed.... and only one showed definite signs of it but she has recovered although she is typically petite. I have another cockerel from the same hatch that is not thriving and has had digestive problems which my gut instinct tells me is Marek's related. Interestingly that broody hatched her chicks outside the hen house, so those chicks were not exposed to Marek's from day one.
I offer supportive care to my Marek's birds and only cull when they are unable/unwilling to eat and drink. It is hard to watch them floundering but I've come to the conclusion that they are not in pain but merely frustrated that their limbs don't work and I usually put them in the "infirmary" (an old sideboard in the hen house with mesh across the front instead of doors, so that they remain in contact with the flock) until they figure it out. One pullet had 3-4 months of supportive care in there last winter when she was pretty much nest bound, but made a miraculous recovery with a bit of spring sunshine and grass. If they are bright eyed and keen to eat, they get a chance to fight it.
I know it is heart breaking when it first hits your flock and losing even one or two birds is a shock, but there are people on here that lose almost their whole flock to it and I consider those to be the ones with the aggressive strain. When my flock first developed it and I started reading up about it, the information was all doom and gloom but after that initial outbreak things have settled down and my experience is that it's not anywhere near as bad as the stuff I read.
Good luck with the rest of your flock and hopefully your experience will be like mine and the problem will settle down after this initial outbreak.
PS. A critical factor in supporting them is keeping them happy and stress free. That's why I don't isolate them and I won't tube feed (not got anything against tube feeding but not for Marek's birds) Having chicken company and competition for food really helps them battle through it. I feed my sick birds whatever they will eat and particularly good treats like scrambled egg, yoghurt and meat or fish. Getting them out in the sunshine onto grass made a huge difference... I put them in a cage so that they are safe until they are more mobile.
Edited by rebrascora - 2/6/16 at 2:59am