Hello99: I am still not sure why you are completely comparing a Milsa D’avida L’ta’ama to a Ma’amid. Can you give the source/link this Rashba so we can see it inside? I would think that Avida L’ta’ama, while operating under similar principles to Ma’amid, would be worse, particularly in this situation. (Especially because I think I recall hearing that we treat a Milsa D’avida L’ta’ama as not Bateil because it’s inherently Nosein Ta’am, not because it’s a Ma’amid.)
Also, I think that you may be mistaken in applying the Shach 87:35 here. He does not say that by all Heter. The basis for something becoming Basar B’chalav is that Derech Bishul Asrah Torah. So you need meat to give flavor into milk (or vice versa) in a way that is a Derech Bishul. There is a separate Issur of a Davar Hama’amid. We say that a Ma’amid is never Battul, so we treat the Issur like it’s there. Thus, when you have a N’veilah Hama’mid in something, it is like a N’veilah. Having meat be Ma’amid in milk is fine because there is no Nesinas Ta’am so it never becomes Basar B’chalav. You just happen to have a food with both meat and milk in it that is Muttar to eat. In this case, however, we are not concerned with something becoming Basar B’chalav. We are concerned with something becoming Assur because it has an Issur D’rabannan of having milk in it. If we treat this milk like a Ma’amid (according to your understanding of the Rashba) and that it never be Batel, then these products now have milk in them, which makes them Assur Mid’rabannan.
(As an aside, I’m not entirely convinced that the bread becomes Assur immediately and you can’t add the Siman after the fact. If your intention is to add a Siman by stamping a letter D or something like that into the bread as soon as it comes out of the oven, I’m not entirely convinced that that wouldn’t work.)