[Coda]

Electron-volts are like light-years. You're not measuring electrons or volts any more than you're measuring light or time; it's a measure of energy based on an electron moving across one volt, analogous to deriving a unit of distance from how far light travels in a year.

"Why not quarks" is because you can't measure quarks directly, because they don't occur in isolation; they ALWAYS occur in bound groups and will instantly transform into something else if you manage to break them apart. Electrons are, as far as we can tell, actually fundamental, instead of being made up of other things.

And yes, your conclusion about rest mass is accurate as far as modern science can tell.