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 is the ruling on someone who possesses gold with the intention of saving/hoarding it, and owns nothing else — is he obligated to sell a portion of it in order to pay the zakāh due on it?
All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah ﷺ.
Zakāh becomes immediately obligatory on cash, and likewise on gold — that which is not designated for a woman's personal adornment— once both reach the niṣāb (minimum threshold liable for zakah) and a full ḥawl (lunar year) has passed over them. It is not permissible to delay payment when it is genuinely possible to pay — meaning, when the wealth and the eligible recipients are both available. It is stated in Mughnī al-Muḥtāj (Vol.2/p.129): "Zakāh is obligatory immediately once it becomes feasible to pay it, meaning when the wealth and the eligible recipients are both present."
The one paying zakāh must discharge the obligatory zakāh due on saved (hoarded) gold either from the gold itself, or from its equivalent monetary value, in the event that no other funds are available — since paying from the gold itself is the original default (al-aṣl). It becomes permissible to pay from other funds instead only because zakāh is built upon the principle of ease and leniency, as stated in Asnā al-Maṭālib (Vol.1/p.366): "When a full ḥawl has elapsed on wealth other than trade goods... zakāh attaches to the actual asset itself (al-‘ayn), and the poor effectively become partners in it — even, for example, [becoming partners] in camels through the value of a sheep [owed as zakāh on them]... Paying from other wealth is permitted only because zakāh is built upon the principle of ease and leniency."
Accordingly: the one paying zakāh must discharge it either from the actual saved gold itself, or by selling a portion of the gold equal to the amount of zakāh owed. He may also borrow money to pay the zakāh, or pay the zakāh in installments before the ḥawl is complete [as an advance payment]. And Allah, the Exalted, knows best.
What is the ruling on the ablution of one who cuts his nails, and is it permissible to cut them before the ritual bath for major impurity (janabah)?
Cutting nails does not invalidate ablution, and it is permissible to cut them before the ritual bath for major impurity. And Allah the Almighty knows best.
What is the ruling on performing ablution (wudu') and ritual bathing (ghusl) with Zamzam water?
It is permissible to perform ablution and ritual bathing with Zamzam water. However, scholars disliked using it for cleaning after relieving oneself (istinja'). And Allah the Almighty knows best.