According to Chofetz Chaim, what was the main mistake of the meraglim? How did Yehoshua and Calev try to deny their words? How can we understand that the Sanhedrin was wrong in this sin? "When we look at it, it is found that the spies mistake is also in us this kind of evil inclination"- how?

ב"ה Shalom Just after the sentence you quoted from Sefer Shmirat Halashon Vol. II chapter 19, .the Chafetz Chaim explains exactly what he means by this evil inclination and basically answers all your questions. What the Chafetz Chaim explains is the following.: Just as sometimes our ego and pride can mislead us, sometimes a false piety and humility can be a fabrication of the Yetzer Hara= evil inclination. The meraglim deduced from the challenges facing them upon entering Eretz Yisrael that they had great battles to face in fighting to overcome the giants and the topographical obstacles of Eretz Yisrael. . To succeed in such an endeavor their evil inclination tilted them to think this undertaking is only for tzaddikim with great merits. They thought themselves how can they who are so unworthy and ridden with guilt of previous sins undertake the conquering Eretz Yisrael with such major obstacles to overcome. Therefore the Sanhedrin was also able fell under the spell of the meraglim with this false piety. So how did Calev quite them down? (Bamidbar 13:30) As Rashi says he said to them "Is that the only thing which Moshe did for us? He also parted the sea for us and gave us the manna." His intention was to say, explains the Chafetz Chaim, "If we had not been worthy neither would Hashem have parted the sea and given us the manna. This way Calev explained to them that their whole perception was wrong and that someone who properly believes in Hashem's promise of Eretz Yisrael would never come to such a perception. All the best