What are the consequences of declaring someone an infidel, believing them to be a polytheist, and then it turns out to be otherwise? And are the Sufi shaykhs infidels?

Excommunicating a Muslim is a serious matter. The danger of it is sufficiently explained by the Prophet Muhammad's (peace be upon him) sayings: "Whoever calls a man a disbeliever, or says 'enemy of Allah,' and he is not so, then it will recoil upon him," and "Whoever accuses a believer of disbelief is like one who has killed him," and "Whoever says to his brother, 'O disbeliever,' then it has returned upon one of them."

Ibn Taymiyyah said: "No one has the right to excommunicate any Muslim, even if he errs or makes a mistake, until the proof is established against him and the clear path is shown to him." Therefore, excommunication is a legal ruling in which reason has no place, and its evidence is textual and definitive.

If you declare someone a disbeliever who is not a disbeliever, you have committed a great error. You must repent, feel remorse, seek forgiveness, and resolve not to repeat it. As for excommunicating Sufi shaykhs, that is not for you. The default status for them is Islam until proven otherwise. And they are not all alike; among them are innovators and extremists, and a general ruling cannot be applied to them.