What happens if you go to Kvatch before beginning the main quest-line?
Solution 1:
Yes, Kvatch is already destroyed at the beginning of the game. You can go into the Oblivion gate, destroy it, and free the town before going to the Weynon Priory. Martin will stay in the chapel after the town is free, until Jauffre asks you to bring him back to Weynon Priory.
Solution 2:
Yes, Kvatch is under attack immediately when you start the game. What happens is that you can save Kvatch from the oblivion gate but you won't find out about Martin's significance until you go and give the Amulet of Kings to Jauffre.