1. re: last wide line in rashi before the second mishna
deebur hamathil: and beit hillel holds since the planks are "battul" and this proves/demonstrates that the ceiling is p'soolah.
what does "the ceiling is p'soolah" mean?
does it mean since he knows the concept of taaseh v'lo min haasui, and he has now negated the ceiling through pikpuk OR netilla.
"The ceiling is Pesulah" means that he understands that the ceiling is not Kasher for Schach in its present state, so once he does Pikpuk or Netilah there is no Makom for a Gezeirah.