How could one call himself a"motorcycle electrician" and not have a meter, much less a test light?

 

It runs happily for 10-15-20 minutes and Trudy don't start, then the solenoid dies...

After almost a year of being turned off and dismantled, I put the cables back where they were before (more or less), but I suspect I did something wrong, maybe I reversed some cables, and now I'm terrified of setting her on fire with an electrical problem.

Why did you take it apart? Taking things apart WILL NOT fix anything.
Especially when it was already working for 10-15-20 minutes.
You only needed to measure battery after it dies to discover that battery is DEAD!!!
We already knew that from previous testing.
Be sure to take photos of before before disassembly, all wiring and parts.
So you can reverse steps in order and put back to original condition.

With modern motorcycles ever since 1947, most problems you'll encountre will be electronics.
Best to learn how to read wiring diagram and use modern 2-way test-light or multimeter.
Do not take anything apart to troubleshoot, only do passive, non-changing, non-interfernce, non-damaging measurements in as-used configuration only.

Have you ever used print maps to plot course to destination?
You only plot course through terrain without tearing roads apart right?
No disassembly of roads needed to reach your destination.

Or used modern GPS maps? Step-by-step directions will guide you from start to destination.
Without needing to tear roads apart...

These are exactly same as wiring diagrams with start location and destination.
Simplest to clamp ground probe of meter/test-light to battery -neg terminal.
Then touch +pos probe to measure voltage at each junction on map/diagram to your destination.
Where power disappears indicates problem is between that junction and previous one that did have power.
Always start at battery as that's benchmark control test that all others will be compared to.

Right-click image -> Open New Image in New Tab to view full-window larger image.
Create step-by-step directions through wiring diagram just like Google Maps.

1. power starts at battery, red wire volts=???​

2. power flows into and out of circuit breaker, red/wht wire volts=???​

3. power flows into and out of ignition switch, key ON, org wire volts=???​

4.​

5. stop switch ON...​

6...​

​

Complete and fill in rest of step-by-step above. Then compare actual wiring on bike.
To ensure it's connected exact same way and that power reaches destination (coils).
Pay particular attention to those two blocks marked with pink arrows.
What are they?
Why are they there?
I'll give you answer at end of this exercise if it hasn't made sense by then.

Many cables have different plugs, so there aren't that many combinations, but some, such as the coil cables, can be inserted in different ways. I can connect the two coils to each other or connect them to two different plugs on the motorcycle's wiring, and I have no idea which is the correct way.

No... look at diagram, there's only ONE WAY to connect coils that is correct and will work to generate spark.
Sure, you CAN plug coil cables and wires to your tyres, but doubt you'd get sparks that way.
Just because you can do something, doesn't mean that you should.

As with pre-flight aeroplane checklist or metabolic tests at doctor's office, things are done in order a certain way for reason.
Don't skip steps, do things exactly as indicated in manual/diagram and bike will run like brand-new right off showroom floor.

You will find problem along way very easily like this example. Might even be same fault on your bike.

https://www.motorcycleforum.com/threads/please-help.256860/

BTW - we are not Superman and can't see electrons moving inside wires (or not moving).
So your pictures does us or you no good.
Better to take those 6 step-by-step measurements you outlined above from wiring diagram.
Problem will jump out at you so obviously, it'll be like lion chewing on your leg, can't miss it.