At any rate, the next step in providing a real wall was mortar into that crack. This took place concurrently with the window patching of the last post and the result was less blurry in person than this picture would indicate. Note also that I've started to patch the cracks in the plaster to the left of the window.
Above the window, the plaster had come loose from the wall, but was still relatively solid, so I used my adhesive caulk to glue it back to the wall. Not a technique that I would recommend for a wall that is intended to be smooth at the end of the process, but for my purposes, a nice white wall that gives some indication of its history, it's probably ideal.
Here's the corner as it appears today. There's plaster in the cracks above the window, I'm using plaster to smooth out the mortar, and the south end of the west wall is almost done; there were some seriously large cavities there, but not really large enough to justify mortar, so I'm filling them in with multiple applications of patching plaster. And obviously, the white paint has already encroached on the corner area.
I'll sand all this plaster tomorrow and see where it needs more fill. Also tomorrow, I hope, will be drywall on the frame section of the south wall (where the garage door used to be). Above the dark beam (that supported a rail for the garage door) the plaster is in seriously bad condition, again having disengaged from the wall and cracked. I'm not yet sure how I'll handle that. Probably more caulk and plaster patch; it's working very well elsewhere.
This corner is going to look great.