Whom is "ve'Gam es Achecha ... Viyelavu eilecha" coming to include?
Rashi: It is coming to include 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 Levi'im).
What is "Viyesharsucha" then referring to?
Rashi: It is referring to guarding the gates and appointing them as treasurers and officers in charge of Hekdesh.
Seforno: It is referring to the Levi'im guarding the Keilim in the Kodesh - denying the other tribes access to them.
What is the phrase that follows "ve'Atah u'Vanecha Itach ... " coming to add?
Seforno: It is coming to place the onus of guarding what is inside the Kodesh Kodashim (the Aron and the Luchos ha'Eidus) upon Aharon and his sons). 1
Seforno: And the next Pasuk teaches us 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 Kodashim and the Kodesh, albeit outside the walls.