Why did the ass see the angel, but not Bil'am?
Rashi: Hashem gave the ass the ability to see it 1 but not Bil'am, who, because he possessed intelligence, would have gone out of his mind had he seen the destructive angel.
In order to save Bil'am's life as is evident from Pasuk 33.
Why was the angel holding a sword?
Rashi: 'Rasha!', he said to Bil'am, 'You are coming against Yisrael using their weapon - the mouth 1 , so I will threaten you using your weapon - the sword!' 2 In the end, he did indeed fall by the sword. 3
Chizkuni: This was Midah ke'Neged Midah, because in Pasuk 29, Bil'am will say to his ass 'if I had a sword, I would kill you!'
In keeping with Yitzchak's B'rachah to Ya'akov, in Bereishis 27:22 - "ha'Kol Kol Ya'akov.". Sifsei Chachamim ? "Shelufah" spells "Shelo Peh" (Yisrael's [weapon] is the mouth).
Mizrachi (citing Bamidbar Rabah 20:13): If not for this reason, why did he need a sword? An angel can kill by blowing (Yeshayah 37:36, 40:24)!
See 31:8.
Why does the Torah not mention what affect this all had on Bil'am's two servants and to the dignitaries of Mo'av?
Seforno: Because when Bil'am's ass went into the field, he abandoned them. Consequently, they did not wotness the episode with Bil?am and the angel. 1
Presumably, Hashem arranged that they should not witness the episode with the ass, which would have turned Bil'am into a laughing-stock in their eyes. Refer to 22:30:1:1**. See also Ramban on Pasuk 33, who maintains that the dignitaries witnessed everything, as doe Rashi ? Refer to 22:29:1:2*.