You did exactly right in having the necropsy done. We've had chickens for years, and, in my experience, either they get sick, or they don't. In other words, we've had a few chickens in the 12 years that we've had chickens, that came down with neurological illnesses...LL or Marek's, who knows. So far, in those cases, the illness seemed to be confined to one bird, except last winter, when we had a few (about 5) very young birds come down with something neuro, one, after the other, during a very hard, cold winter. Point being, I think birds whose immune systems are compromised some way are more likely to get sick, and others with healthy immune systems will not. If it were my flock, I'd keep them on vitamins for a while now. Oxine, although expensive, is an excellent disinfectant, and a little goes a long way. You can put an eighth of a cup or so in a small spray bottle mixed with water, and spray everything in sight, including the chickens themselves. (Just lightly mist the chickens). You can also put a few drops of Oxine in their drinking water, and add it to wash water if you are washing/ sterilizing their feeders/ waterers. It's 10 X more powerful than bleach. We get Oxine at firststatevetsupply.com. Another thing that MIGHT help is 1st Tuesday Lime, it's nice because you can use it in the coop and run, and you don't have to worry about it harming the chickens, but it will kill/ sterilize bad stuff pretty well. Again, it's pricey, but a little goes a long way. Lastly, make sure your flock is as healthy as you can get them, that' they are as parasite free (internal and external parasites) as possible, and, like I said, vitamins certainly won't hurt. The rest of your flock might get by OK. Probably will. Also, a lot of people worry about the surviving birds being carriers. Whether that's true or not, I don't know, but again...see the first part of this (very long) post.
And yes, especially if you're getting new birds from an unfamiliar source, they need to be in quarantine. We keep our new birds quarantined for three to five weeks. It also helps them to acclimate to their new surroundings and get used to the flock and is a lot less stress on them and also your existing flock.