Mason jars don't hold pressure- they hold a vacuum. As soon as any pressure would build up from carbonation, the pressure should be released. That sounds like it should prevent bottle bombs. Campden doesn't kill yeast, so you can't depend on it to stop fermentation.
I'd leave it in the fridge for now, since it's possible that fermentation hadn't finished. It'll take longer to condition that way, but should be safest.
I recommend taking a look at the stickies at the top of the cider page- that should help you from having any problems in the future.