1)
Why does it mention the Zekenim coming?
1.
Malbim: They and the Kohanim were primary for carrying the Aron. The Kohanim carried it in the presence of the Zekenim.
2)
Must Kohanim carry the Aron?
1.
Radak: (Normally, Leviyim carry it.) In three places, Kohanim carried it - when Yisrael crossed the Yarden, when they surrounded Yericho, and when they returned it to its place when David fled from Avshalom. Why did they not count now, entering it to the Kodesh ha'Kodoshim? Perhaps it is included in returning it to its place.
