Carrying The Aron

The Rambam in mitzvah 34 lists a mitzvah for the kohanim to carry the aron. He says although in our parsha the command is given to the Leviem that was only since there were so few kohanim at that time but for the future it is a command for the kohanim. There are two questions on the Rambam. One question is how can the nature of a mitzvah change? And why does the Rambam hold that the mitzvah of carrying the aron applies as a permanent mitzvah but the other vessels are not included, if the source of the mitzvah, כי עבודת הקודש עליהם בכתף ישאו refers to all the vessels of the Mikdash? 

The Meshech Chachma and Briskor Rav (letters) both point to a possuk in Ekev as the source of the Rambam. The possuk (10:8) says בָּעֵ֣ת הַהִ֗וא הִבְדִּ֤יל ה אֶת־שֵׁ֣בֶט הַלֵּוִ֔י לָשֵׂ֖את אֶת־אֲר֣וֹן בְּרִית ה. This possuk singles out the aron as having a command to be carried. From there the Rambam derives that the mitzvah of carrying the aron is a permanent command. The Rav explains that they are two distinct laws, one is a law in carrying the Mishkan and that is a command to the Leviem in Naso. The possuk in Ekev tells us that there is a din in how to carry the aron when it is moved. He adds that is why the din of carrying the aron is not a command to the Leviem (that was only when does as a means of transporting the Mishkan) but is a command to the kohanim (and the Rambam would understand the possuk in Ekev as a reference to kohanim.) There are two issues I have with this interpretation. One issue is that if the Rambam is sourced in Ekev why does he not cite that possuk in the Sefer Hamitzvot, and he only cites the possuk of בכתף ישאו? In addition, we are forced to say the possuk in Ekev is not referring to Leviem but Kohanim (which is not like Rashi on the possuk, see Ramban.) And even if we grant that, why does the Rambam not say that is why the kohanim carry the aron and instead says a severah?    

The Rebbe (L.S. volume 28) says the Rambam understands from the Midrashim that line up the importance given to the family of Kehas to be counted first since they carry the aron which contains the Torah, that it is the aron which is the most significant item carried by Kehas. What defines the carrying of the aron as a permanent mitzvah is that carrying the other items is a command on the leviem, a command on the gavra how to carry the kelim. The aron requires carrying because the cheftzah of the aron demands carrying directly. With this we can understand how the mitzvah is transferred to the kohanim for it is not a command on the gavra to carry the aron, it is a command for the aron to be carried, who fulfills the act of carrying is a mere detail in the law.