Is the one offering the sacrifice liable if its meat spoils?
Praise be to Allah, and prayers and peace be upon our Master the Messenger of Allah.
If the meat (of the sacrifice) spoils due to the negligence of the one offering the sacrifice in preserving it, or due to improper storage, then he is liable to compensate for the portion due to the poor (which is estimated as half a kilogram of meat). If the sacrifice was a vowed (mandatory) one, then he is liable for all of it.
However, if he was not negligent, then there is no liability upon him, because its ruling is the ruling of a trust (like an item left in someone's care). And Allah Almighty knows best.
When is a child instructed to fast?
A child is instructed to fast at the age of seven if they are capable of fasting and have reached the age of discernment, by analogy to prayer. They should be encouraged but not forced, so they can become accustomed to it. It is obligatory for their guardian, whether a father or another, to instruct them.
What are the Sunnahs and etiquettes related to the slaughtering of an animal?
In the Name of Allah, and may peace and blessings be upon our Master, the Messenger of Allah.
1-Avoid terrifying the animal before it is slaughtered.
2-Do not slaughter the animal in front of others of its kind.
3-Offer water to the animal before the sacrifice.
4-Sharpen the knife thoroughly prior to slaughtering to ensure the cut is made as quickly and cleanly as possible. This is in accordance with the statement of our Master, the Prophet (peace and blessings be upon him): "Indeed, Allah, the Almighty and Majestic, has prescribed excellence (Ihsan) in all things. So when you kill, kill well; and when you slaughter, slaughter well. Let each of you sharpen his blade, and let him put his sacrificial animal at ease." (Narrated by Muslim)
Do not begin skinning the carcass until you are absolutely certain that the animal has completely passed away.
Slaughter cattle, sheep, and goats while they are lying down on their left side, facing the Qiblah (direction of prayer). Leave the right leg untied so the animal can move it, helping it rest more easily during its final moments. For camels, however, the preferred method (Nahr) is to slaughter them while they are standing up, with their left knee tied. And Allah the Almighty Knows Best.
Is it a condition for the mosque where I‘tikaf is performed to hold Jumu‘ah prayers?
No, it is not a condition for the mosque where I‘tikaf is performed to have Jumu‘ah prayers. However, if a person vows to perform continuous I‘tikaf, and Jumu‘ah occurs within that period, then they must observe I‘tikaf in a mosque where Jumu‘ah is held so that they do not break their continuity by leaving for Friday prayer.