An honest answer to your very reasonable questions is, I don't know. I am not a physicist. I am just speaking from experience.
But let's theorize... and talk about practicality.
Let's say I'm in Japan and you are in US. Let's say our antennas are using ground as "virtual the other side". Because of huge distance and the fact earth is not a perfect conductor, we can't reasonably assume they are connected in the same sense they are copper wire. Yet, it works. Plus, a radio wave is a EMF emission. Electrical and magnetic field in 90 degrees difference. Ground takes no part in this type of propagation. Using the common "virtual the other side" theory.... they are more like a vertical dipole with no ground. I've done this many times. Using railing of a high rise building, many times using cars, outside shed, sheet pan and so on. They all can be tuned to have no reflection. Worked well, too. Aircraft antennas on small planesare aircrafts are vertical antennas often with metal sheet on inside of fiberglass body.
I don't think of antennas as capacitively coupled. If that's the case, magnetic field will not be involved. But low band antennas are often wires on ferrite rod. I think they magnetically couple. So I don't know.
The reason why connecting to neutral side of outlet works is that long wire in the circuit works as ground plane. A reference for how vertical antenna works against and kind of forms "the other side". Also, wavelength of AM broadcast band are long enough that length of this wire is less critical. I assume you are aware neutral side is connected to earth ground at your breaker panel and at the pole? Not an ideal situation but practical albeit risky.
A very ODD thing I used to do. Remember rotary telephone? Remember the finger stop? THAT works as an excellent antenna. This never made any sense to me as these are not connected to anywhere. But worked. Explain that!
I think of antennas as piece of wire with something to push against. I don't necessary subscirbe to virtual antennas in the ground theory. It explains many antennas but not all. It may be easier for you if you think about how dipole works and ground plane works. Even in absense of ground side, that side is coupled to ground somehow - very lightly and inefficiently.