To whom does "u'Shemartem es Mishmeres ha'Kodesh ... " refer?
Seforno: It refers to the Kohanim, who are obligated to guard the Mishkan within the planks.
Sifri: It is a warning to Beis-Din 1 to warn the Kohanim to perform the Avodah properly, because when the Avodah is performed properly, it prevents punishments from coming upon the world.
See Torah Temimah, note 5.
What are the mplications of the word "ve'Lo Yih'yeh Od Ketzef"?
Rashi: It implies that there will not be anger again, like there was earlier - in 17:11.