1)
Why does it say "va'Yitz'aku" regarding Midbar and seas, and "va'Yiz'aku" regarding a Choleh and prisoner?
1.
Radak (13): They are the same; therefore, sometimes one is written, and sometimes the other.
2.
Malbim: Tza'akah is greater than Za'akah. Also, regarding Midbar and seas, it says "Yatzilem", and regarding Choleh and prisoner it says "va'Yoshi'em", for Hatzalah is greater than Yeshu'ah; the former are in greater danger.
2)
What is "ba'Tzar"?
1.
Radak: It is like ba'Tzarah (in the affliction).