Unexpected home maintenance

So we are installing our gas fireplace. However the plumber is unable to tell where the gas line is (because we are a condo), hence he'd have to cut a 12" x 12" hole in the wall to check. I mentioned we were considering tearing out the mantle to either rehab it or replace it because the wall was buckling.

The wall the mantle is attached we assumed, incorrectly of course, that it was plaster chipping. Nope. We took off the top piece of the mantle, look inside the wall and determined that the drywall is coming off the studs. Eek!

Because we haven't taken off the whole piece we don't know if something is wrong with the studs or if it's just the drywall pulling off. Hence now we are going to have to get a quote for rehanging the drywall. We are considering doing this ourselves, which we've done before, however DH is worried because this drywall is easily pre-WWII. So we might open up the wall and see things we're not expecting because of the age of the house.

Hence, I am going to get a quote about the rehanging the drywall. I might rehab the fireplace because it does match our mouldings, but DH refuses. He doesn't understand why you would want to save an old mantle painted over 20 times. But we live in an area where people want the charm of an old mantle. Ugh, this is going to take a professional telling him what I'm saying for him to listen. That should happen on Thursday when the fireplace guy comes in to examine the mantle and determine if we should rehab or replace it.

This repair is absolutely necessary. We can't have our pipes burst again from the cold because of inadequate heating of our bottom floor. We left the furnance on to heat to 50, but because it has to pipe down 4 stories (attic to third floor), it was working 100% of the time and still having trouble heating to just 50 on the bottom floor. We also used space heaters but it did not help. Thus we feel our only alternative is to install a gas fireplace and attempt to heat the bottom floor as a mini-furnance (though it's radiant heat). It's better than nothing.

We didn't realize this was such a huge problem. Cost will probably be I'm guessing 2x what I budgeted of $4k. So hopefully we can get away with it costing $8k?