Okay, so the glue-up of the broken key apparently worked.
As we all learned in high school shop class, the hardened glue will actually be stronger than the wood surrounding it, so I believe I have effected a fix. Or redemption. Or just a fix, we'll see.
More importantly, I discovered shortly after hammering all of the balance rail pin hole slots that there was a conspicuously large gap between a couple of the balance rail pins (keys 35-38 to be exact). I had noticed this in previous photos and was anxiously waiting for someone to call me on it, but no one ever did.
It turns out I missed drilling the balance rail pin holes for keys 36 and 37. This is a huge drag because I drilled all of the holes when the keyboard was a single piece of jointed wood. This means all of the holes were perfectly aligned, I just needed to cut the keys apart.
Needless to say, it was a major pain in the rear-end to get keys 35 through 38 aligned precisely enough that I could be confident drilling the holes in this post hoc manner was going to yield good results. There was a lot of swearing and painter's tape involved, but I finally got things aligned and drilled. Not fun at all.
The good news is that all of the slots are completed and I can move on to filing and sanding the holes to clear them of any errant detritus left from the slotting process. After that, I will begin the process of cutting the sharps from poplar and getting them glued on.