You are right, Marek's can live on your property for years, it's a disease you likely will have to deal with for a long time.
As for the respiratory illness, most of those don't live in the environment for that long, but sometimes even after culling, cleaning, etc., you could still get more birds that are sick - especially if certain diseases are more common in local flocks. There's no guarantee.
http://edis.ifas.ufl.edu/vm011
Most with diseases in their flocks do the best they can to manage symptoms like you have been doing.
Marek's does complicate things and it's a virus so antibiotics do not help with it's symptoms. Supportive care (for Marek's) includes vitamin therapy (B-Complex), keeping stress levels low, see that the bird is eating/drinking well, stay on top of parasites like worms and lice/mites, etc.
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
Having respiratory disease in addition to Marek's complicates things for you. I can't imagine what that's like. If the birds do respond to antibiotic treatment and you mentioned bubbles in the eyes, then most likely you are dealing with Mycoplasma - but really the only way to know for sure would be with testing.
Managing symptoms is usually what most do if they have MG in their flock as well. Denagard is often used to help with symptoms and given to all the birds for 3-5 days each month - this may be something you want to look into. Denagard will only treat MG.
As for Tylan50 - yes, there is a lot of dosing information out their and it can be quite confusing. I try to go by something like Plumb's vet drug manual whenever possible. So....
The dosing information I have is from Plumb's Vet Drug Manual - the dosage for Tylan (Tylosin) is 10-40mg/kg given 3 times a day for 5 days.
I would go with a mid-range dosage of 25mg/kg - that translates to 0.227 mg per pound of weight. Personally I would round up to 0.25mg per pound of weight so it's easier to calculate (you have plenty of wiggle room on dosing - remember we are looking at mid-range on dosing here).
After all that mess of me running my mouth - this is what I would give - 0.25mg per pound of weight orally 3 times a day for 5 days.
IF you were only giving an injection once a day and this girl is very sick with the respiratory symptoms - then yes, I believe if she were mine I would give another round of Tylan (orally, 3 times a day for 5 days).
Now...another thing, just in case you are not aware. A LOT of antibiotics are hard to find. Tylan50 and 200 have been taken off the shelves it seems. Tylosin (which is Tylan) powder can be used but you would need to order it. Lots of places have short stock or no stock, you have to look daily (hourly?) It would be a very good idea to go ahead and order yourself some Tylosin (powder) to keep on hand since you do seem to have an ongoing problem here.
I'm very sorry that you are dealing with all this. Keep us posted on how she's doing.
