Only bread likely to be eaten with meat.
Where do you get that from? Milchig bread is assur as is, despite intentions or ex-post-facto simanim.
“it would have the status of not batel”
Not true. The Issur b’DiEved on one who was MEvatel Issur is ony a Knas.
According to the TZ”TZ that the issur to be m’vatel issur applies here, why should’t the knas apply here as well?
Mixing fleishig rennet with milk itself is not an issue of Ma’amid if the meat is intrinsically Muttar. Bread intended to eat with meat cannot be worse than the meat tself!!!
How is that nogeia here?
The Shulchan Aruch and Rema in YD 134:13 DO Pasken the Rashba.
Make up your mind how you want to pasken, but don’t change just to prove your point against whomever you happen to be disagreeing with on a given day.