Why does it say "ki'R'oschem"?
Malbim: You will see two changes from usual. Normally, two Degalim went before the Aron, and Leviyim carried it. Now, the Aron will be in front, and Kohanim will carry it.
Why does it say "ha'Kohanim ha'Leviyim"?
Rashi: All of the Kohanim are from Levi, i.e. from Amram. In 48 1 places, Kohanim are called Leviyim.
Radak: Until now, Leviyim carried the Aron. Now Kohanim carried it, to sanctify it more for the miracle. There were two other times when they carried it
Rashi cites Midrash Rabah. Perhaps this is a printing mistake. Whenever this is mentioned in the Gemara (e.g. Yevamos 86b) and Midrashim, it says '24'. (PF)
Radak: I.e. when David fled from Avshalom (Shmuel 2 15:29). Perhaps when they brought it to the Beis ha'Mikdash is included in 'when they returned it to its place.' Or, it was not counted, for it stayed there permanently. Rashi (33b), Tosfos (33b) and the Malbim (5) explain that 'when they returned it to its place' means to the Beis ha'Mikdash. Perhaps at the time of Avshalom is included in this.
Was it normal to go after the Aron?