The whole thing is an abstraction. The nucleus isn't actually tiny ball shaped things mashed together, but rather cloudy stuff which would probably not be identical if we could actually see them. The quarks that make up protons and neutrons are considered elementary particles and identical, but they don't move around much unless energy is used to split them.

The electron however is an elementary particle that moves outside of the nucleus and can move from one atom to another. So the hypothesis is that if we could follow one electron from the big bang to the end of the universe, and this electron could move both forwards and backwards in time, it would potentially be enough with just one.

It probably doesn't hold up very well, but it's an interesting thought experiment.

No, electrons are much smaller than protons, which are slightly smaller than neutrons.

You'd have to ask John Wheeler, which would be difficult since he died in 2008.

A big part of quantum mechanics is the fact that matter can show wave-like behaviour, which sort of breaks a bunch of "rules" that we have from classical physics. This only is relevant if we're looking at stuff at a teensy tiny scale.

Someone else has already mentioned that electrons are a fair bit smaller than protons and neutrons (around 1840 times smaller) and this means they tend to have a smaller momentum than protons or neutrons, which means they have a larger wavelength, which was easier to measure experimentally. That's likely why electrons were a part of this theory, because they're small enough that they're sort of a perfect way to study the idea of things that are both particle and wave, but also neither. In 1940, quantum mechanics and particle physics were super rapidly moving fields, where our knowledge hadn't congealed much yet. What was clear was that electrons get up to some absolute nonsense behaviour that broke our understanding of how the world worked.

I like the results of some of the worked examples here: https://www.chemteam.info/Electrons/deBroglie-Equation.html , especially the one where they work out what the wavelength of a baseball would be (because that too, could theoretically act like a wave, it would just have an impossibly small wavelength)

TL;DR: electrons are smaller than protons/neutrons Smaller = larger wavelength Larger wavelength = easier to make experiments to see wave-like behaviour from the particle Therefore electrons were useful in figuring out how the heck a particle can have a wavelength and act like a wave

Maybe, because we can measure the number of protons and neutrons with an ion accelerator? I don’t know if the something similar can be done with electrons.