[lisper]

>You end up gluing together two of the electrons and these two electrons will accelerate at the same rate as the single electron.

Not really. You have swept under the rug the fact that it's really hard to glue electrons together. And if you were to actually find a way to do it, you would have to add so much potential energy to the system that it's inertial mass would increase dramatically. In fact, two electrons that were actually "together" (whatever that might actually mean for a quantum particle that obeys the Pauli exclusion principle) would have a mass orders of magnitude higher than two electrons separately.

You probably want to talk about ions in an electric field, which you can "glue together", but then it becomes rather obvious that they don't all accelerate at the same rate.

The equivalence principle is not the only thing that distinguishes gravity from other forces. There is also the fact that there is only one gravitational "charge" and it's mutually attractive. (Gravity is also many orders of magnitude weaker than all other forces.)