Excellent! I had this same situation a few months ago. Coach723 really helped me. You've got great help here already but if the situation continues to be complicated, I recommend getting help from Coach. I treated mine for Coccidiosis and for sour/impacted crop. She almost made it. Unfortunately...
Start all on Corid and give sick one a direct Corid dose. Make sure that you're not giving anything extra like vitamins. Just water and feed.
https://www.backyardchickens.com/threads/corid-oral-drench-instructions.1211991/
Depending on which Hatchery the store sources (you could call them and ask,) you're right that some vaccinate for Marek's, some do not. I agree with you that it would be strange to have only one suffering from Mareks.... You have nothing to lose, to try right away, treating for Coccidiosis...