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.
Is it permissible for the children of a deceased father to settle his debt from the Zakah (obligatory charity) money due on them?
It is impermissible to use the Zakah of one`s money for settling the debts of the deceased. However, children should settle the debts of their deceased parents out of filial piety. And Allah Knows Best.
What type of illness that permits breaking the fast in Ramadan?
It is an illness that, if one fasts, there is a fear it may lead to their death or cause unbearable hardship.
Which holds greater virtue: The Udhiyah (Sacrificial Offering) or giving its value in Charity (Sadaqah)?
In the Name of Allah, and may peace and blessings be upon our Master, the Messenger of Allah.
Offering an Udhiyah (sacrificial offering) holds greater virtue than donating its monetary value to the poor and needy. This is because the sacrifice is a prominent, manifest ritual among the sacred symbols of Islam. Allah Almighty says: "That [is so]. And whoever honors the symbols of Allah - indeed, it is from the piety of hearts." (Al-Hajj: 32)
Furthermore, there are numerous authentic and explicit Hadiths highlighting its immense reward. The Prophet (peace and blessings be upon him) consistently offered this sacrifice, as did the Righteous Caliphs after him. And Allah the Almighty Knows Best.