1)
What is the meaning of "va'Yitzku"?
1.
Radak: They poured [into bowls for the men]. The Yud from the root is omitted; only the Yud due to the conjugation is written.
2)
Why did they say "Maves b'Tzir"?
1.
Radak: [The Paku'os] are bitter.
2.
Malbim: (a) The ones who already ate will be harmed. (b) The others cannot eat.