How can the Zakat threshold for past years be estimated, and is it obligatory to pay Zakat again if it becomes clear that the threshold was reached in a different month than the usual time?

Repentance is obligatory for delaying the payment of zakat. Regarding the zakat on salary and the nisab, I refer to previous fatwas. If the money reaches the nisab in Dhul-Hijjah, and the zakat has not been paid, then it is obligatory to pay it in Dhul-Hijjah. If it was paid in Shawwal, then it is obligatory to pay the zakat for Dhul-Qa'dah and Dhul-Hijjah, and then the hawl (zakat year) becomes established in Dhul-Hijjah. It is permissible to pay zakat in advance, and if it becomes clear that the money in Dhul-Hijjah is greater, then it is obligatory to pay the zakat on the surplus. This applies to previous years; if the beginning of your hawl was in Dhul-Hijjah and you were paying in Shawwal, then it is obligatory to pay the zakat on the surplus for Dhul-Qa'dah and Dhul-Hijjah for each year.