Of course I am not able to comment on ALL models and explanations (mentioned in this thread) you are referring to, but of course a few general comments are in order.When I first read the above opening question in this thread my original thought was(and tentatively still is) that within experimental error the charges on the electron and proton are numerically equal and that charge is conserved and that total charge is zero,these things being based on observations and measurements.In other words the answer to the question is that it is a "fact of nature" based on experiments.
Now this thread has been going on for a long time with people trying to answer the question by referring to various theories ,models and equations.Most of what has been written goes enormously above my present knowledge but at least I have been able to scan and get a rough feeling of what the discussions have been about.
Throughout the discussion I have had the uncomfortable feeling,and at present it is only a feeling,that some or all of the theories referred to are based on the relevant experimental observations such as those listed above.If that's the case is it legitimate to use a theory based on experimental observations in order to explain those observations?Surely it is the observations that should inform the theories and not the other way round.
Anyway,when I get time,I will try to research the origins of these theories and the experimental evidence on which they are based.In the meantime if anyone can enlighten me I will be grateful.
There is a rather satisfactory theory called "the standard model of elementary particle physics" which subsumes our present knowledge regarding electro-weak and strong interactions as relativistic quantum field theories. Currently there is no evidence (at least not for experimentally accessible energies at colliders including the first LHC results) that this SM is at odds with known experimental facts.
[There are hints from cosmology and astrophysics that dark matter may exist which would be certainly a strong hint towards physics beyond the SM; there are numerous questions that cannot be answered withing the SM, e.g. the values of the masses and coupling constants of SM particles; but even if the SM is expected to be replaced some day by a deeper theory explaining some of the SM ad hoc inputs, nevertheless the SM passed all direct experimental tests as of today]
Afaik all above mentioned models and explanations are nothing else but certain applications, interpretations or formulations of the SM. That means we tried to show how the SM explains (or at least motivates) the experimental fact that the electric charge of electron and proton are numerically equal. There was and still is a strong and fruitful interplay between theory an experiment that led over decades to the formulation of the SM. Therefore you should not worry about the observations and the theoretical explanations we discussed.
[coming back to DM and a possible explanation via SUSY: I don't think that the arguments presented here would change so much when taking SUSY into account]