Standard wiring practice duplex receptacles, both screws used?

Really? Generally that only occurs when electrolytic capacitors are installed with incorrect polarity.

"A properly-connected capacitor shouldn't have current flow in a DC circuit, so it should not warm up"

Caps on the output of a rectifier are subject to ripple currents, but that's not dependent upon the input voltage.

Many vintage radios predated widespread AC availabilty and ran from batteries.

My stewart-warner requires an A (1.5v) battery and two 45v B sized batteries (the instruction diagram shows a third 45v B battery in pencil, probably written by my Grandfather). The A Battery powered the filament, the B batteries provided the plate voltage.

Which is within specification.

Never heard of the 'leakage current'?

Any time the voltage goes up the current must go up in most devices. There are a few in which 'negative resitance' is possiable. Mainly some simiconductors.

By how much does a 5% decrease or increase in the supply voltage affect the leakage current? The operational leakage current is very small, and is unlikely to be significant between 120vac and

I'm still not convinced that a voltage swing within the standard -5/+5 percent mains voltage tolerance would significantly affect any capacitor in the voltage conversion circuit (e.g. filter caps on the power supply), or any capacitor in the low-voltage DC side (where the plate voltage of 45VDC becomes 47VDC comparing your nominal 115VAC to 125VAC)

At 125 volts not much, but on a radio with a power transformer you can be talking 600 volts - and on a :leaky" cap that adds up pretty quickly. "reforming" catalitics by ramping the voltage up slowly with a Variac can often salvage old caps

Caps used in electronics like this are typically rated for voltages well above the actual designed circuit voltage. Like if a circuit is designed for 30V, you're probably going to see like a 50V cap. Caps in old radios are likely to fail and if one is nearing it's end, certainly giving it a 5 or 10% higher voltage could push it over the edge.

What kind of radio has a power transformer with 600V?

The older capacitors had to be formed at the factory similar to charging a battery. The factory would often form them for about 5 to 10 % above their rated voltage as they would deterate some when not in use. After they were used in devices they would reform to the near the lower voltage they are being used at. Say a typical old radio capacitor was rated at 250 volts and used on a 200 volt application ( typical of old tube radios) the capacitor over the years reforms to slightly over 200 volts. Hooked to a 125 volt line like I have here and the transformer boosts that up to 240 volts and the capacitor stands a good chance of going up in smoke. Capacitors are also rated for temperature. Many for

It was 110 when I was little and if it's 120 now, it's because of inflation, and that's probably because of the Democrats.

Years ago when I used to measure it, it was usually 117.