To whom is "Makeh Re'eihu ba'Saser" referring?
Rashi: It is referring to someone who speaks Lashon ha'Ra 1 (which is spoken in secret - Targum Yonasan).
Targum Onkelos: It refers to someone who strikes his fellow-Jew in secret.
See also Ba'al ha'Turim Dibur #1 and Dibur #2.
Bearing in mind that the last 'Arur' is a general one that pertains to all the tribes (Refer to 27:26:1:1), to which of the twelve tribes did Moshe not insert an 'Arur'?
Rashi: Corresponding to Shimon, since he did not intend to bless him when he blessed the other tribes before he died.
Targum Yonasan: These twelve covenants were made on Sinai, repeated in the Ohel Mo'ed and repeated again in the Plains of Mo'av - thirty-six B'risos all in all - See Na'ar Yonasan.