In the case of a shor that has been nigmar din and is awaiting s'killa that beomes mixed up with other animals that are innocent we learn that they are all stoned or locked up and allowed to die.
Why don't we say according to the opinion that they are put in a cell and allowed to die that we go basar rov and that the first one to die was the one that was nigmar din and the others are then puter and should be released?
We do do this with issure v heter by mixtures. So why not here?
Your question touches upon two issues: (1) Bitul, and (2) Tolin. If there is one Shor ha'Niskal which is Chayav Misah, and one of the bulls in the mixture dies, we should follow Rov and say that one of the Rov (i.e. a Mutar Shor) died. On the other hand, the concept of Bitul should work to say that the Asur Shor becomes Batel in all of the Mutar ones *mid'Oraisa*, while mid'Rabanan there is still an Asur Shor among them. Hence, when one of the bulls then dies, we should apply the principle of "Tolin" to say that it was the Shor that was Asur (*mid'Rabanan*) that died.
The answer is that we do not apply Bitul here, because "Ba'alei Chaim Einam Betelim" -- a living animal cannot become Batel. Regarding the rule of "Tolin" see Zevachim 74a.
D. Zupnik