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.
What is the ruling of Islamic Law on following the actions of the imam in prayer and how this following is achieved?
Praise be to Allah, and peace and blessings be upon our master, the Messenger of Allah.
The follower (ma'mum) is required to follow his imam in the actions of the prayer. This "following" (mutaba'ah) means that the follower performs each action of the prayer after the imam has begun it but before he has finished it. For example, the follower bows (in ruku') after the imam has reached the position of bowing, then rises after the imam has risen. The Prophet, peace and blessings be upon him, said: "The imam is only appointed to be followed, so when he says the takbir, say the takbir; and when he bows, then bow; and when he prostrates, then prostrate" (agreed upon, i.e., reported by both al-Bukhari and Muslim).
It is stated in Mughni al-Muhtaj (1/505): "Among the conditions of valid congregational prayer (iqtida') is following the imam in the actions of the prayer... meaning that following the imam is obligatory in the physical actions of the prayer, not in its verbal utterances... Complete following (kamal al-mutaba'ah) is achieved when the follower's beginning of an action comes after the imam's beginning of that same action, while the follower's beginning of the action precedes the imam's completion of it" — end of quote, with slight paraphrasing.
And Allah, the Exalted, knows best.
Is it permissible to eat from an Udhiyah slaughtered on behalf of a deceased person?
Praise be to Allah, and prayers and peace be upon our Master the Messenger of Allah.
It is permissible to eat from the sacrifice (uḍḥiyyah) that is offered on behalf of a deceased person. This is the madhhab (school of thought) of the Hanbalis. In this case, the heirs take the place of the deceased as if he were alive, with the same rights to eat from it, give it in charity, and offer it as a gift.
It is stated in Maṭālib Uli al-Nuhā (Vol.2/P.472): "Offering a sacrifice on behalf of a deceased person is better than offering one on behalf of a living person, because the deceased is unable (to perform deeds) and is in need of reward. It is to be treated like a sacrifice on behalf of a living person in terms of eating, giving charity, and giving gifts." And Allah Almighty knows best.
What should one who doubts the validity of his ablution and performs it frequently do?
Whoever doubts the validity of his ablution after finishing it, his ablution is valid. This is because the default is the validity of the ablution, and because doubt after finishing an act of worship doesn`t affect its validity. And Allah the Almighty knows best.