Particle physics isn't really my specialty, but I think you're wrong on all counts. Gravity affects light, so particles going backwards in time wouldn't sudden

Particle physics isn't really my specialty, but I think you're wrong on all counts. Gravity affects light, so particles going backwards in time wouldn't suddenly experience gravity as a repulsive force. The curvature of spacetime is what matters. (Light experiences zero subjective time.) The strong interaction affects protons and neutrons, not electrons or positrons. "Zero effect" is implicitly time-invariant. As to the weak interaction, it is mainly a (reversible) "given inputs, get outputs" process. Both β+ decay and electron capture have an analogous process for positrons. The catch is that when you swap signs on the electron, you have to make corresponding changes to the rest of the equation - e.g. if there's a proton for the electron version, it needs to be an antiproton to work for a positron.