Just a guess, but when the bolts came loose, some of the gasket stuck on the manifold and some on the engine.
An exhaust manifold leak really won't cause much of a problem, unless it's pushing hot gases onto something that will melt. I've wrapped a leak with aluminum foil to deflect the gases away from sensitive stuff. I've stuffed a leak or crack with aluminum foil. Aluminum foil and a hose clamp work pretty good for sealing a crack on the tubing (for a while). Check the hot gases are blowing away from sensitive stuff. Check the wiring (CPS etc.) in the rear of the motor. Copper foil works better, but it's harder to find.
A good way to tell what's going on, in a cold climate, is to take a look early in the morning, right after starting the motor. You can see the leaking exhaust gases, just like your breath on a cold morning. A flashlight and a welders mirror comes in handy.
Header bolts are notorious for backing out. Non ferrous bolts seem to stay put better. I always use a dab of Loc Tite, not really what it was designed for, it seems to get kind of crispy, but seems to work OK.
