Welcome to my world of Lotus Electrics
Joking apart, I get the frustration and having been in similar places I'll throw in a few ideas on the off chance. My wiring is UK spec so quite different from yours but there might be a few comparisons we can draw.
1. Bullet connectors - I have them in the front & rear compartments and wonder if they could be the cause of power out of the fuse box but not at the headlamp ?
2. Looking at the later federal diagram there's a connector block marked just after the column switches which is different from my car. It might be worth checking for 12v out of that connector if it's under the dash, it might have been knocked during final assembly.
3. But my biggest problem on any of the old Lotus cars has usually come down to earthing which works ok until you load it up and then it unreasonably goes AWOL. I've slowly added separate earth leads from every light, front & rear, all grouped together to form an earthing loom. No multiple bullet connectors, wherever it's been possible it's a single wire to earth with no connectors other than either end.
In the front I use a stainless bolt where all the front leads gather and that earths directly to the chassis through the front closing plate (also stainless) and hence to the front cross member. That main earth also carries the electric fan and headlight/fan/etc relays which are also located in the front compartment.
At the rear it's under the luggage tray and bolts to the chassis leg for an earth. It's been so long now that I can't remember if that's standard or not but I do recall adding lots of extra wire and relays (weight !) to the car.
Brian