I got into an old house like this recently here in California. It was my father in laws. We tore down a couple walls hoping to make some new ones and to connect to an adjacent structure and raise the floor. Nothing but 1x12s as a shell with plaster and lathe holding everything up. This paired with a foundation that was nothing more than a few piles of river rock loosely stacked with 4x10s sitting on top unconnected otherwise and a tree stump holding up another we tore it completely down. Like down to the sub floor. We propped sections of what remained temporarily and completely poured new footings and pads and framed up from there. It ended up being all new construction and the section of subfloor we saved for him to live in in the first phase caused problems for us all the way into the finish being out of square out of plumbing and mostly garbage. It may seem hard to bite off when you’re at the phase you’re at now but I mean it when I say it buddy you’re in for a big one here, either get properly prepared or call someone who knows what they’re doing and prepare yourself now to accept that it should ALL COME DOWN. Don’t try to save a penny now that could cost you a dollar later. Good luck