Wow wMw, I'm in the exact same boat over on Kingsley Ave. Trust me, you don't want to know whats in those walls!!
Our place is a combo of knotty pine/masonite walls and I too am in the process of repainting. Just the other day I was looking at the same cracked joints you've got and thinking how they look the same as they did when I was a kid, and debating about re-taping and muding them (cause I've got to be smarter then the last guy who painted, right?). Then I thought better of it and had kinda settled on running a bead of silicone cock over them and blending as best I can. The big issue with that is you can see where the tape itself is starting to lose its adherence to the masonite.
So other then re-sheet rocking I'm dieing for options too!