Why did the donkey see the angel, but not Bil'am?
Rashi: Hashem gave the donkey 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?
Chizkuni: This was Midah k'Neged Midah. [In verse 29,] Bil'am says 'if I had a sword, I would kill you!'
Why does it not mention what happened with Bil'am's two servants or of the dignitaries of Mo'av?
Seforno: When Bil'am's donkey went into the field, it abandoned them. 1
It seems that Hashem arranged that they should not witness the episode with the donkey, which would have turned Bil'am into a laughing-stock in their eyes (EC). Refer to 22:30:1:1**. See also Ramban on Pasuk 33 and refer to 22:29:1:1* - Rashi, who maintain that the dignitaries witnessed everything.
How come that no further mention is made of Bil'am's two servants or of the dignitaries of Mo'av?
Seforno: When Bil'am's ass wondered off into the field, he lost them. 1
Permanently it seems - so 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 22:29:1:1*. See also Ramban on Pasuk 33, who maintains that the dignitaries witnessed everything. Moreover, they are mentioned later in Pasuk 40.