Hello99,
A few quick points:
I think we need to distinguish between the issur l’chatchilah of ein m’vatlin issur, and the issur b’dieved (you’re correct that I misused the term “not batel”, because for some it’s muttar, but the effect on the mevatel and the mi shenisbatel bishvilo is that it’s treated as not batel).
Concerning b’dieved, Rav Schachter clearly writes that according to the shittah of the Tzemach Tzedek, it would be assur – “?????? ?? ??? ?? ????, ???? ??? ?????? ????? ???????, ??? ????? ???? ??? ????? ??????”.
Anyhow, if we assume that it is batel, I think based on 99:5, we can safely conclude that at worst, that the OU is mistaken, anyone relying on it (including Thomas) would not be more than a shogeg , so we should limit our discussion to the perspective of the certifying agency. I believe that the OU holds that putting a hechsher on a product makes any ingredient be considered mevatel l’chatchilah (although I don’t think Thomas is Jewish owned).
Clearly, according to the Tzemach Tzedek, it’s assur l’chatchilah, so we cannot throw out the normal rules of the issur of “ein loshin”. In other words, I don’t understand what you wrote, “Since it is a completely separate Sugya from the original Din, it has its own set of Halachos and it cannot be presumed that the rules of the full-fledged Issur apply.”, since we are indeed discussing l’chatchila, which is not a separate sugya.
Concerning the Rashb”a, aside from applying it l’chatchila (even if not b’dieved), the Pri Megadim (M”Z) on ma’mid writes that avidi l’tama is worse than ma’mid.
As far as your final point, I still don’t think you need to feel obligated to be moch based on a “significant” view, if it’s not absolutely halacha p’sukah. You probably should have written in that original post that it’a an added point. I do agree, though, that the ingredients might be outright tarfus and that most processed foods need a hechsher.