I have found a partial but acceptable solution, hinted at earlier. I am convinced that this is ripple current. I’m still unsure of why I hear it ONLY when I touch the metal casing of the LCDi3, nor why it disappears when I touch other metal in the circuit - either an open lightning connector or an iPhone or iPad case.
When using the earliest and smallest AC power from Apple, it was slightly reduced. I still conjecture that an overbuilt AC charger that fully filters ripple current, such as the Topping P50 might work. I invite @SebastienChiu to let me know if their 5v iDefender filters ripple current. Given its form factor, I don’t see how it can hold and adequate capacitor.
I bought Zendure’s 100W bundle promotion, figuring that I could always use both additional AC and portable power:
The “SuperTank” battery is heavy, but smaller than I anticipated. Dimensions are given as 4.7" x 2.8" x 1.6" (119mm x 73mm x 40mm). Weight is a half kilo, or just over a pound. Both items are well-made. The charger gets warm when charging the SuperTank in fast mode at 100W, but not alarmingly so. It gets slightly warm when charging at 18W.
Both items are well made and sleek. I think that the company caters a bit to people who do backpacking and hiking. The battery has an LED numeric indicator and a power switch. The kevlar-coated USB C to C 100W cable looks sturdy enough to hog-tie a wild boar. Shipping is from China, and takes about 7 to 10 days. Although they have a store on Amazon, I ordered direct because there is more selection and additional deals on their website. After ordering they supplied me with this kickbooster link, which promises me that rewards will pile up for me if you buy from it:
https://zendure-supertank-2020.kckb.st/c27c9bd9 https://zendure-supertank-2020.kckb.st/c27c9bd9
Now for the results
I had hoped that the high-quality “SuperPort” charger might reduce that ripple noise, but it does not. It is no better and no worse than several other chargers, including larger genuine Apple chargers.
I had hoped that even while charging, the SuperTank battery would somehow provide filtering so that I could listen indefinitely. 
Unfortunately, that also does not work. The hum still gets through.
However, charging using the battery alone, the hum goes away. All is blessed quiet. And that battery has a honking big capacity, so that even if I let it charge my iPad all night while listening, there is plenty of juice left in it. I could probably go for several days. I pronounce this at least a partial SOLUTION.
Due to consumption of liquids in the evening, actual indefinite listening times are not possible.