It isn't if we can't find the electrons going backwards, which we generally can't. Nowhere near enough positrons in the right places. I consider it a fun example of the sort of thing that modern physicists should be familiar with, so it's useful both historically and for didactic purposes, but as a conjecture it appears to be simply false.

No, it could be disproven. Take a high-energy gamma ray, and have it undergo pair production, creating an electron and a positron. Capture both in the same Penning trap, then let them annihilate.

This would result in a loop that is disconnected from any other electrons, showing that the electron/positron in the loop is distinct from other electrons.