Hi Poppy, I'm back.
I know how devastating it is to be told that you have Marek's Disease (MD) in your flock. I lost approximately 2/3rds of my flock to the disease before it burned itself through and out of my flock of 40+ birds.
Yes the tumors are associated with MD. I lost every rooster to tumors before the age of 18 months. One lived to be 20 months old but he was the exception. Others died from ocular Marek's and neurological Marek's Disease. All survived the initial infection with the exception of one which succumbed at 9 weeks to scissor paralysis/MD. But they survived the initial infection only to die of secondary MD before the age of 2yrs.
I have no idea where it came from but rest assured, you did nothing wrong. It's just there and it is everywhere and it is a killer you cannot fight or win the fight that you are having with it.
Seriously. I wish I could give you good news but you will lose more birds before it is done with your flock but as it gets worse, it will get better. In my case the deaths/culling of suffering birds, ended suddenly and without warning.
I was so glad when it did. I lost one hen who was brooding chicks when they were just 4 weeks old also about 5 years ago. She was my last suspicious death as she was fine when I locked her and her chicks up and I found her dead in the nesting box the next morning. Her single chick, a little Old English Game Bantam cockerel was bereft with grief and I became his surrogate mom. He still greets me at the coop door and will occasionally bring me gifts of a blade of grass, a piece of food, a piece of plastic he found in the run. he's a doll and I love him to pieces.
The best advice I can give you other than what was in my article is dig your heels in, hang on and wait for it to end. It will end and the birds you are left with will be your truly resistant birds that you can breed resistant chicks from. I won't lie to you. You may still lose chicks you hatch, ones' that aren't resistant. I decided to breed for resistance by adding Egyptian Fayoumis which are genetically resistant to Marek's disease along with vaccinated bantams because frankly I love bantams, wanted some and this was the only way I knew to bring new birds into my flock without them curling up their little toes and dying on me.
I do know what you are feeling right now. I have one little hen who has cancer in her belly, probably reproductive organs who is three years old now and I'm expecting her to pass over the summer if not before. Is it MD? I don't know honestly. She is the only bantam who is having problems right now so I'm going to think that she just is at the age where hens have reproductive cancers/infections pop up.
But you and your flock can survive MD. It won't be easy. You will be tempted to throw in the towel but don't unless that is what your heart is telling you to do. Because every time my brain told me to quit, my heart and my husband (interchangeable 

  ) told me not to.
I tell members that I have posted to about MD that they are not alone. Lots of people here are dealing with it. It's a club of sorts that nobody wants to belong to, yet unfortunately they find themselves members. Please if you have any questions, feel free to ask me. I may not have all the answers but by golly I do have the expert experience.
Blessings,
Becky