Good catch! I see now that I did incorrectly wire the gigaohm resistor. Maybe that's the reason why the picoammeter is totally not working right now.
Uh, you've twice referred to a 10megaohm resistor, but, unless I'm overlooking something, in your schematic it's 1 Megaohm. Based on context, I'm assuming that's what you meant.
The yellow wire isn't actually sharing a standoff with the megaohm resistor. Maybe it looks differently in the picture. However, there is a slight, unintentional glue bridge between the black wire and the blue wire's support offset, so I'll clear that away during the repair.
In case you're interested, the green LED I'm using is a CRE, model 630-HLMP-CM1G-350DD. Costs about 10 cents each in quantity 10 from mouser. I had previously tested it as being dimly visible even at just a 500na current drain, provided you can look at it dead-on (which is when it's narrow optics helps some). Here I'm running it at 10ua because the orientation isn't so great. I had earlier thought of having the LED do double-duty by also illuminating the trim-pot, as a navigation aid for screwdriver insertion, but it turned out the hole is big enough and the trim-pot is high enough that room light alone is enough to see where the screwdriver needs to go.
Lest I lead someone down the wrong path, I should mention that I'm not thrilled with the aluminum enclosure I'm using. I purchased it from amazon, and in pictures it looked like plain aluminum, but in reality it has some kind of non-conductive silvery coating on it. Therefore, as belt and suspenders, I'm going to tap each of the four pieces that constitute the shell of the enclosure and wire them together, similar to what an electrician might do, to guarantee that they really are all electrically connected. The drill-tap for doing that should arrive tomorrow. If I were to do the project over from scratch, I'd probably use a die cast aluminum box instead of this coated semi extruded aluminum box.