Charge is defined by Faraday's law
The standard answer is Gauss law, but it allow for charge being any real number e.g. half of electron ... while in nature charges are integer multiplicities of e.
We repair it by interpreting field curvature as electric field, this way Gauss law counts topological charge/winding number - this way enforcing it to be quantized as in nature.
Gauss–Bonnet theorem - Wikipedia
en.wikipedia.org
Yes in the > 100GeV far field scattering limit
No, the standard model starts with QED e.g. to get agreement with ~10^-6 eV Lamb shift https://en.wikipedia.org/wiki/Lamb_shift