Are the same cards available for a particular arena to all players?
Are the cards specific for a particular arena or do they vary for each player (as some higher arena cards have a question mark on it).
If they are fixed, what cards can be earned in each arena?
All cards are the same for everyone. Every card can only be unlocked on a given arena and this doesn't vary. Here are all the arena specific unlocks. The cards can be viewed here: troops, spells and buildings. God knows why they wouldn't show them to us, or allow us to click on them and view their stats. My best guess is they are trying to preserve the element of surprise...