OK. It has been a week and a half, and I finally got an hour to pull this thing apart. To start, the wheel spins freely and quietly as you would expect. But it does show being loose as you can hear and just see and feel the knock as you rock the thing back and forth when it is up in the air.
When I pulled the axle, there was no leaking under the cap at all, and the splines on both ends look very good.
When I pulled the drum I was surprised that the lock screw came out fine, and the drum came right off without argument. However, underneath you can see piles of grease that has come out of the hub against the back of the stub axle. Oddly, none got on the shoes or drum.
The tab washer looked to be bent in an unconvincing way... and as such, I think this is why the lock washer was only hand tight. The bearings and even the seals look fine. I think the grease leaked out merely because the hub came loose. And since it hadn’t even got to the shoes, or even dropped down to the lower part of the backing plate, I believe this is a new issue with few miles on it.
My thoughts right now are to simply clean it up and put it all back together to see what happens. I experimentally locked the hub down and put the wheel back on (sans brake drum or axle shaft) and spun it around and rocked it and didn’t get any indication that it was loose or broken in any way. One spin, and the wheel rolled on its own for almost 15 seconds without a sound.
I am going to switch to disc brakes anyway. If nothing seems broken, why not just run it till I get the parts for the swap? I think I may even get lucky and be able to properly adjust the brakes without the hub being loose as it was.
I’ll get some right stuff and brake cleaner, and see if I can put it back together tomorrow.