Must a specific person be excommunicated if a scholar excommunicates him, and does failure to excommunicate him fall under the rule of "whoever does not excommunicate the disbeliever is a disbeliever"?

The principle "whoever does not declare a disbeliever to be a disbeliever is himself a disbeliever" applies to disbelief that is known by necessity to be such, like the disbelief of original disbelievers. It does not apply to a Muslim who falls into disbelief due to ignorance or a misconception before proof is established. The scholarly endeavor to declare a specific individual from those affiliated with Islam to be a disbeliever is subject to acceptance and rejection, especially if others disagree with it. The general populace is not obligated to delve into these matters or to rule an individual to be a disbeliever. The disagreement regarding the disbelief of Al-Hajjaj ibn Yusuf Al-Thaqafi is evidence of this. An error in pardon is better than an error in punishment. Not everyone who falls into disbelief has disbelief befallen them. Issuing a ruling of disbelief is not a trivial matter.