Re: extremely uneven floors
Massive central fireplaces were generally built on solid deep foundations. While the rest of the foundation settled the fireplace structure didn't and the framing around the fire place got hung up on it. While jacking and leveling can be done by a competent DIYer once the proper methods and cautions are learned if you feel uncomfortable with the project a professional is an excellent investment.Some paster cracking is probably inevitable.
I would recommend a full inspection by a structural engineer who will be able to advise you of the needed work and an estimate of the cost.