The conclusion about the influence of Montezuma among the Mexican people that is best supported by the excerpt is:
He was still very influential as the legitimate ruler of Mexico.
This is evident from Cortés's plan to bring Montezuma to the Spanish quarters, which implies that Montezuma's presence and authority as emperor would provide the Spaniards with a significant advantage. The suggestion that the Spaniards could use him as a "tool for the government of his own kingdom" indicates that they recognized his status and influence among the Mexican people as a legitimate ruler, and sought to leverage that to secure their own safety and control.