You write in the summary:
5. Objection #1: a Beraisa explicitly says ...
6. Objection #2 (Beraisa): If most of tongue was removed
Why are TWO beraisos needed? If the point is to demonstrate that Rebbi considers the lashon external, just bring ONE beraisa.
Boruch Wolf, Baltimore, MD, USA
The Gemara questions what is Rebbi's opinion with the tongue concerning the Mum of slaves - first there is a proof from Beraisa 1 about Haza'ah and then from Beraisa 2 about the Mum of Bechor. Bringing a 2nd proof definitely helps because it prevents the possibility that Beraisa 1 of Haza'ah has its own private reason which prevents it being a source for us - and now that there are 2 proofs we are more convinced that the tongue is considered external for the Mum of slaves.
All the best,