Great post! Well thought out and also highlights na tamad ang typical gitarista, trying to solve all these problems using a noise gate, when it should be a last resort.
Share ko lang, as in most cases as the TS highlighted, wiring is the culprit. It's the fact that signal, shielding, and power, all share a common name for their return path. Ground. Send a signal, you need ground to complete the loop. Shield a guitar/amp, you need to tie it together catch all the stray noise. To power something up, you need also a ground for it to complete the electrical circuit.
The problem is because they share a common "term" to describe the return paths, most people assume they could tie them all together and not have issues(or not too big of an issue). But fact is they can all add up. So it's best if you can to observe how each part of the signal chain is assembled, from your guitar wiring, pickups, pedals, cable, amp, and their individual components, transistors, tubes, and see if they practice making the distinctions.
Signal, shield, and power grounds should be ideally considered as three distinct streams, actually power should be considered sewer line, just to emphasize how bad it is for the three to mix if you want a pristine noise free signal. These streams all eventually meet up in one river, but they should be collected properly.
I think most guys understand this in guitar wiring, but not know how bad it is in cheaper pedals, power supplies, and amplifier components. All the more reason to appreciate guitar, pedal and amp gutshots!
