Some more work done. The wall where the window is is now exposed to the exterior sheathing on the top. It is damp enough that when I ran the scraper over it, water squeegeed to the top. I had to pull out the old balsam wool insulation, because having it get wet leaches the fireproofing out of it.
Some of the studs have some rot. Luckily, they're right next to the chimney, so they're not crucial, and the rot is not through-and-through, but surface. It's repairable.
I have to pull the rest of the lath off of that wall and let the air get to it to dry it out. The center wall above the mantle is plaster on top of brick, and is in pretty good shape, but for the edges. I'm probably going to frame it over and route cabling there for the TV after all is done.
The window wall on the right hand side needs the same treatment too.
Really, I should do this with all the external walls and get rid of the balsam wool entirely. Aside from getting rid of the flammable insulation and replacing it with non-flammable and more efficient insulation, it also gives me a chance to rewire and get rid of the knob-and-tube wiring, and once and for all fix the badly patched plaster.
Not gonna happen soon.