Hi
So sorry for your losses. Two birds in a day must have been so distressing!
I'm just reading through this thread for the first time and I've been suspecting Marek's from the first page although the symptoms were by no means typical of the disease. So few cases of it are diagnosed because there are no hard and fast rules with it. I commend you for getting a necropsy and being tenacious enough to keep asking questions until you get a result that makes sense.
I was with
@KikisGirls that this was not your fault regarding giving them fermented feed. I would however be interested to know what the feed was that you were using (pellet, crumble, grain mix or mash) and how you were feeding it ie free choice dry from a feeder as well as some fermented, scattering a set amount on the ground or feeding from a pan. Fatty Liver Haemorrhagic Syndrome is a growing concern and a significant proportion of people who have lost birds to it and had it diagnosed via necropsy, have been feeding a grain mix or whole grain feed. I think if you are using such a feed then it needs to be significantly limited rather than fed "ad lib" but there are still concerns that some birds may not get a balanced ration due to the ability to pick and choose favourite components from it rather than a micronized pellet or crumble where each particle is nutritionally balanced. People choose these grain mix feeds which are usually organic and expensive, because they believe they are giving their hens the best, but the reverse can actually be true. Not sure if this is the case in your situation since I didn't see any reference to the type of feed or how you feed it other than it being fermented. I'm just interested in collating information and also encouraging you to consider a switch to pelleted feed to reduce the risk of Fatty Liver within your remaining flock.
As regards other birds not showing signs of Marek's, it is not the sort of disease where all birds experience an outbreak or symptoms at the same time. The disease can have long dormant phases within infected birds and an outbreak can be as subtle as a bird having difficulty keeping one eye fully open or as dramatic as suddenly floundering on their side in the splits unable to get up, or just die quite suddenly as you sadly saw with your Brahma, Ollie. Some birds may recover from outbreaks and some will not. Outbreaks are usually triggered by stress. What stresses one bird may not stress another. There are many situations where you cannot prevent stress, like the surge or hormones at point of lay or drop in hormones at moult, prolonged stormy weather, or predators prowling around at night unsettling them.
Some strains of Marek's are much more aggressive than others. I have one of the milder ones in my flock, so my experience is slightly different from Nambroth's who wrote the article on Marek's Disease that was linked. I get some birds survive outbreaks and go on to have a good quality of life. Supporting their immune system is one of the most important aspects of that, so a good quality poultry vitamin supplement like Nutri Drench or Poultry Cell is helpful and access to grass whenever possible. I'm also a fan of fermenting feed and whilst I don't feed it exclusively, I like to give them at least some each day to help support their gut flora.
It is quite common for sick birds to be infested with lice, so don't feel too bad about them having lice or odd that the remainder of your flock do not appear to have them. Sick birds do not dust bath and preen themselves like healthy birds and lice take advantage of that and thrive on them. Many people tend to find lice on a sick bird and assume the lice are to blame but generally it is the other way around.
I think it is likely that you will see the odd case of Marek's from time to time in the future in your flock but hopefully this initial outbreak will be the worst. Some birds definitely are resistant to it but there really is no way to know other than the fact that they survive year in and year out without getting symptoms. Your vaccinated birds should survive regardless but there are concerns that the vaccine allows the virus to mutate, so more virulent strains develop as a result, so vaccination is not necessarily the best way forward, but if you already have a very hot strain of the disease, then vaccination may be your only option when adding to your flock.
I hope that it is a long time before you have to face another such loss.
Bear hugs to you! 
