What does the word "Ach" come to preclude?
Rashi and Ramban #1: It precludes someone who does not do Teshuvah from a Kaparah.
Ramban #2: It does preclude anything, but comes to substantiate the statement, as if it had written 'Achein' - in truth.
Every 'Ach' and "Rak" in the Torah comes to preclude something. See Ramban who elaborates.
What does "ve'Inisem es Nafsoseichem." incorporate?
Targum Yonasan: It incorporates a prohibition against eating, drinking, washing (bathing), anointing oneself and intimacy.
Halachically, it is only the first two that are min ha'Torah. The other three are only mi'de'Rabbanan. See also Ba'al ha'Turim.