I am so sorry
@Eggsandbeyond
@microchick is right, hang in there.
I think "it depends" really fits, there's a chance your older birds won't show Mareks symptoms, there's a chance they will fight it off, there's a chance they will get it but not show it and may or may not have a shorter lifespan. It depends on how virulent your strain is.
I may have told you already, I tested 5 birds, and 3 came back negative for MD and only the 2 silkies tested positive. I know a lady who had ALV in her flock, she found that her flock didn't get the ALV if they were exposed after I think it was 16 weeks old. This is her experience, but if the chicks you bought came with ALV, from that information, your older birds may be ok. I don't want to give you false hope, but I also don't want to leave you devastated either.
My opinion on the testing, do it if it will help settle your mind on what you need to do.
If your birds are vaccinated for Mareks, maybe those two pullets weren't?