prefab fireplace failure.

[jeff47]

4. You need a new chimneysweep

This part makes no sense:

"well, now the brick part of the box is failing. sweeps said it is no longer safe (although there is nothing flammable behind it) and needs to be replaced, all the way to the roof. $8k."

The "brick part" of the box is just for show in the box itself. There is no "brick part" going to the roof. There's a metal pipe going to the roof. Any bricks on the outside of your chimney are mostly cosmetic and to protect from rain/snow

Unless your original installer did something insane like not connect the fireplace to a flue pipe.

And as a sibling post said, you would be well aware if you had a chimney fire. They are spectacular. If your chimney was not recently acting like a giant blowtorch, you haven't had a chimney fire.