Who is "ve'Gam es Achecha ... vi'Yelavu eil'cha" coming to include?
Rashi: It includes the B'nei Gershon and Merari - to join 1 the Kohanim in preventing the other tribes from coming close (to perform the Avodah that is allotted to them (the B'nei Gershon and Merari).
What is "vi'Yesharsucha" referring to?
Rashi: It refers to guarding the gates and appointing them as treasurers and officers in charge of Hekdesh.
Seforno: It refers to the Levi'im guarding the Keilim in the Kodesh - denying the other tribes access to them.
Tamid, 26b: It refers to assisting the Kohanim to guard the gates - the Kohanim guard above, the Levi'im, below. 1
Tamid (Ibid): Because as far as the Levi'im performing their own Avodah, the Torah discusses in Pasuk 4. See Torah Temimah, note 1, who elaborates.
What is the phrase that follows "ve'Atah u'Vanecha Itach ... " coming to add?
Seforno: It places the responsibility of guarding what is inside the Kodesh Kodshim (the Aron and the Luchos ha'Eidus) on Aharon and his sons). 1
Seforno: And the next Pasuk teaches that, when all's said and done, the Levi'im are carrying out the service of the Kohanim in that they guard both the Kodesh Kodshim and the Kodesh, albeit outside its walls.