1)
Why does it say "Kichesh Lo"?
1.
Radak: He was a false Navi; he lied to him.
2.
Malbim: He was Machchish (contradicted) the Navi's Perush of the Isur.
2)
Why did the elder think that the Navi would heed him and transgress his own Nevu'ah?
1.
Malbim: His sons told him that initially, the Navi said only "do not drink water" without the word "Sham", and do not return "ba'Derech Asher Halachta" without the word "Bah". He understood that one could explain that there is no Isur to eat and drink after the Shelichus, nor to backtrack, just the Navi is stringent. The elder gave a lenient Perush of Hashem's command.
