1)
Is this only for a Navi who prophesizes for Shalom?
1.
Rashi (from Tanchuma Vayera 3): If a Navi prophesizes for Shalom and it does not come, he lied - "Lo Ish Kel vi'Ychazev" (Bamidbar 23:19). This does not apply to me, for I prophesize punishments. We can say that Hashem retracted [Radak - like He did regarding Ninveh].
2.
Radak: A prophecy for good, even if it is conditional, it will come, but if the people sin, [afterwards] the good will depart - "v'Rega Adaber Al Goy" (18:9) 1 .
3.
Malbim (7-9): Yes. Nevu'os of punishments are so people will repent. Nevu'os of good are to establish a Navi to be a Navi Emes. You are not established to be a Navi Emes until your words will be fulfilled!
