Hi Splash, If you still have brake light, ignition switch is not the problem.
Your problem is distribution of power. Start by cleaning all the connectors inside the headlight shell. But first with headlight hanging out setting on fender, Turn on key & wiggle around wires. Turn bars lock to lock & wiggle around wires where they pass by head tube. Observe lights etc.
Test light on coil power will go dim or flash off if you loose power there while testing. So you can catch it with peripheral vision where it could be overlooked with volt meter.
As I recall you only have the single horn, no relay. Power comes from wires to ammeter. So the crimp is bad, or wire may be bad. Wiring diagram can be followed to trace wires, but it doesn't always show every snap connector.
Poor ground will do the darndest things. Volt drop is best test. But again wiggling red wires & removing all ground nuts/bolts, cleaning & apply dielectric is advised.
The Brown/white wire from ignition switch to headlight switch needs a careful look from ignition switch to headlight switch.
Ohm meter is an important tool. However it can't find volt drops reliably. You need to do volt drop test on each effected circuit. That takes understanding of each circuit & a large test load. Even then depending on the nature of the intermittent faults it may not show the fault.
I agree a total failure would be much easier. Thing is, you may have several problems.
doesn't seem to loose charge, as was stated, alternator is not cause of these problems.
You are most correct installing harness may not cure problem. I've learned over the last 50 years, good diagnosis leads to good repairs.
The saying, "it's better to be lucky than good" is most true!! However 99% of the time it takes a lot of effort to diagnose the root problem. I'd recommend you do web search on automotive pages on how to volt drop a circuit. Volt drop is a very valuable skill for any Triumph owner.