Why did chariots and horsemen (cavalry) accompany Yosef to bury Yaakov?
Ramban (to 49:31): Because he foresaw that Esav would cause trouble. 1 Ha'amek Davar - Only as much as needed went. This is unlike Ziknei Pharaoh - all of them went due to honor.
Seforno: Because they considered Yaakov a warrior. 2
Malbim: It was due to Yosef's third elevation (refer to 50:7:151:1), to be second to the king; they were ready for war. A tradition teaches that they fought with Esav and his sons, and they seized Tzefo ben Elifaz alive, for he wanted to protest against burying [Yaakov] in the cave.
Why does it say "va'Ya'al" (singular)?
Ha'amek Davar: Yosef caused them to ascend with him. They did not go to honor the Mes, or to honor Yosef, rather, because Yosef commanded, to guard the Rabim from any damage.