Apparently, according to Rebbi Shimon, the passuk (La’asos) would have been applied (in the initial stage of the Gemara) to purposely rubbing off the nega; why would this be allowed?
What was the Gemara’s connection between Rebbi Yossi who prohibits the witnesses to travel, since they are presumed to be unneeded – to our case of Millah where he is completing a Mitzvah?