Good news about Bill. Couldn't tell what you were displaying as a "slotted nut" there. Each axle gets two; you can see them in the center of the driver if the builder didn't putty them over. That is where you want Loctite (and putty).
The main crankpin, as I recall, is held the same way, with a slotted nut, from the back side. The eccentric is held in place with a small nail. Being a little loose is not cause for alarm, unless the crankpin itself is loose.
Looks good - at $100 it is a true O Scale bargain. Usually $250.