What does Rebbi Shimon do with the words "es hagomal"? Why doesnt he interpret any es's?
(a) According to Rebbi Shimon, the word "Es" is normal Hebrew syntax and not extraneous. Even if the Pasuk could have been written without that word, the word is not considered extra - along the lines of "Dibrah Torah k'Lashon Bnei Adam" (Bava Metzia 94b and many other places).
The difference between Rebbi Shimon and those who say "Lo Dibrah Torah k'Lashon Bnei Adam" is that the latter refer exclusively to places where the Torah uses a word with the same root twice in succession (e.g. Ish Ish, or Ganov Yiganev), while Rebbi Shimon refers exclusively to the use of the word 'Es'.
Similarly, we find an argument among the Tana'im if the letter 'Vav' should be considered extra and used for a Drasha or not (Menachos 87b and elsewhere). This argument is also along the line of 'Dibrah Torah k'Lashon Bnei Adam', except that it applies exclusively to the letter 'Vav'.
(b) In an answer prepared for Kollel Iyun Hadaf in Bava Kama 41 http://dafyomi.co.il/discuss_daf.php?gid=21&sid=20&daf=41&n=4 , Hagaon Rabbi Yakov David Homnick shlit'a, suggested that various Sugyos argue whether ultimately Rebbi Shimon decided not to learn anything at all from the word 'Es', or whether he just decided that sometimes 'Es' is a Ribuy, and sometimes it is used for another Derasha (which he doesn't specify).
Rabbi Mordecai Kornfeld
Kollel Iyun Hadaf