If electrons and positrons are identical except for their opposing charges, what was the nature of these particles before the EM force separated out?
If the electromagnetic force did not exist before it froze out, how could there have been a meaningful distinction between matter and antimatter?
$\begingroup$ Neutrinos and antineutrinos have no electric charge. This means that there is more to antimatter than having the opposite electric charge from matter. The premise of your question — “electrons and positrons are identical except for their opposing charge” — is flawed. $\endgroup$– GhosterMar 14 at 0:45
In the mainstream model of particle physics , the standard model, the charge of particles is axiomatic.
I say "at present" because because this might be different in the future, if deviations are found in experiments and observations, which is the way theories in physics expand.
In the Big Bang model that is used to fit cosmic observations this is still axiomatic, particles have zero or $+/- 1/3$(quarks) or $+/-1$ .
All the plasma in the various stages of symmetry breaking is assumed to be a soup of charge and neutral particles. (the quantum gravity era unknown and modeled with effective theories).
Electric charge existed before electroweak symmetry breaking. See this answer. It didn't have the special role before EWSB that it has after, but you can still have particles with "what will later become" electric charge, and antiparticles with the opposite value of that quantity.
Also, there are charges in the Standard Model other than electric charge – weak charge and color charge – that are also different in antiparticles.
Third, it's possible in quantum mechanics to have distinct particle species with exactly the same properties. You can tell that two particles with the same properties aren't the same species by their statistics. There are no known examples in nature, but the three fermion generations are almost an example, except that they have different masses.
Fourth, it's fine for a particle to be its own antiparticle (the photon is, for instance).