Why was the exact date of Laylat al-Qadr concealed?

The wisdom behind this is to encourage Muslims to strive in worship throughout all the nights of Ramadan, or at least in the last ten nights. By doing so, they will earn the reward of observing Laylat al-Qadr as well as the reward of worshiping on other blessed nights.

If the menses lasted for over than 9 to 10 days, considered menstruation?

If the duration of the bleeding does not exceed fifteen days, it is all considered menstruation, even if the usual pattern changes.

What is the ruling on performing Tarawih in sets of four rak‘ahs with only one Tasleem?

It is not permissible to combine more than two rak‘ahs with a single Tasleem in Tarawih prayer. Whoever does so, their prayer is invalid. Shaykh al-Islam Imam Muhammad al-Ramli (may Allah have mercy on him) stated: "If a person prays four rak‘ahs of the Tarawih prayer with a single Tasleem, it is not valid if they did so intentionally and with knowledge [of the prohibition]. Otherwise, it becomes an 'absolute voluntary prayer' (nafl mutlaq); because it is contrary to what has been prescribed." [Nihayat al-Muhtaj, Vol.2/P.127].

What are the conditions governing the portion that must be distributed to the poor from a voluntary uḍḥiyyah (sacrificial offering)?