hate to say it, but as long as being underground eliminates all threats from monsters, I’m not really in favor of making it more workable to live underground indefinitely, there should at least be some monsters accumulated on the surface to deal with when you go out for food.
Note I’m not saying it should be impossible to build an impregnable fortress, just that it should require you to build it rather than just walk down some stairs 
also, a muffler shouldn’t decrease smoke output, just noise.