On SANHEDRIN 16 in the POINT BY POINT OUTLINE, point 1) (i)
>>Question: After having learned that a false Navi must be judged by 71, he should learn from "Hazadah-Hazadah" to Zaken Mamrei that 71 judges are required to kill him!<<
My question is: is this question simply a request for info regarding the zaken mamreh? Or perhaps it is an attack on Reish Lakish by stating that if RL were correct that the Gezerah Shavah "Davar-Davar" is the source that a false Navi must be judged by 71, then a zaken mamreh should also be judged by 71 by means of Hazadah-Hazadah and since zaken mamreh is judged by only 23, the davar-davar must not be correct.
Why does Rashi need to say that we would add to the mishna that ZM also needs 71? It was the absence of ZM in the mishna that refuted the original Hazadah-Hazadah to teach false Navi gets 71.
Reuven Spott, Yerushalayim
This is an attack on Reish Lakish, as the Mishnah did not state that a Zaken Mamrei is only judged with 71 (see end of Rashi DH "v'Lehedar"). If this would be correct, it should have stated that, implying that it does not hold he is killed with 71. Therefore, the Gemara asks, Reish Lakish must not be correct.
Your first question is the answer to your second question. Rashi needed to say this to show that the omission in the Mishnah is proof (according to the Gemara's question) that Reish Lakish is incorrect.
All the best,