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?
Not that it would mean directly rubbing it off. It would mean doing something else with the understanding that it will happen. The whole concept of Eino Mischavein is that you aren’t doing this deed. This applies even when you are aware of the strong possibility that it will happen. By a P’sik Reisha we say that you can’t exclude that act from your intention.
In Mila he is not either doing a Mitzva anymore. It is now a matter of Zeh Keili that attaches it to the Mitzva. Rashi describes the case of Nir’e Be’alil as also being a Mitzva, although it is unnecessary at this point. This is similar to Zeh Keili where there is a Mitzva but it has no actual gain Legevoa.
I think that we are not actually trying to prove that Reb Yosi will hold this way. He might personally attribute more significance to Zeh Keili. The point is to find a Machlokes Tanaim that our Tana holds like one of them. By the Machlokes of Rebbi Yishmael ben Rebbi Yochanan ben Broka, the Machlokes what about Zeh Keili, as we learned earlier. Now we are saying that our Tana is that Shita. On this we say perhaps Rebbi Yishmoel would agree in our case. Meaning, that Machlokes does not pertain to our Mishna.
In the Machlokes of Nir’e Be’alil our Tana is decidedly like Rebbi Yosi, that there is no Chilul Shabbos once it is not Letzorech. We knock this away as well, because his Shita might only be not to allow Chilul Shabbos while our Tana is merely extending Chilul Shabbos.
why are we assuming that according to R’ shimon the posuk will permit the actual direct taking off the tzoraas, when the gem. just right after asks according to the other braissa what R’ shimon is going to do with the posuk in hava amina. I don’t know why the gemorah dosent ask on this braisa what R’ shimon needs the posuk.
regarding the second question it seems that there are two approaches the gemorah went about to deal with this shita . one was that will assume that continuing a mitzvah afterwards is not part of the mitzvah, rather you are doing it for the mitzvah but it’s not aa act of a mitzvah, if thats the case R’ yosi is a similar idea. Than the gem. differentiates by saying that here it is a mitzvah because continuing a mitzvah goes into the original mitzvah and is part of it if thats the case the gem. than says its similar to the case of the lechem hapanim.