One important point missing from the above discussion: according to Rav Berkovits, the Vaad Arba Haaratzos was kovea that eid echad neeman doesn't apply when the person is making money off of the food. Those all require a hechsher.
(He says Yerushalayim was not meshubad to the vaad, which is why until very recently, there were places like Hocker's butcher shop that had no hechsher at all.)
Also, as has been expressed in a number of ways in the thread, Rabbi Berkovits commented thus: it is all very well that eid echad neeman - but A) is the person a kosher eid? You certainly cannot say that he is if you don't know anything about him, and B) what, precisely, is he testifying? Does he know anything about issur vHeter? Not everyone does. What does he consider kosher among the commercial products he buys?