Am I permitted to perform more than a prayer within single ablution?
All perfect praise be to Allah, The Lord of The Worlds, and may His Peace and Blessings be upon our Prophet Muhammad and upon all of his family and companions.
You are permitted to pray as many as you could within one ablution and to recite Quran so long as your ablution isn't nullified. And Allah Knows Best.
Should the Zakah (obligatory charity) giver tell the poor recipient that this is the Zakah of his money?
No, he shouldn`t. But, the Zakah giver should make an intention in his heart that it is the Zakah of his money when paying it to the poor. And Allah Knows Best.
Is it permissible to offer Salah (prayer) on behalf of the sick who is on his/her death bed, and is incapable of offering it?
It`s impermissible to offer Salah on behalf of the sick, or the dead.
What is the ruling on taking a loan from an interest-based bank (a usurious bank) to buy an apartment for housing, noting that I am renting at a high amount relative to my income, and I am an employee? Also, considering that the conditions of Islamic banks are strict, and I cannot obtain the full amount from them, is it permissible to take a loan to purchase an apartment or not? May Allah reward you.
Usury (interest) is prohibited by Islamic law, and the Messenger of Allah, peace be upon him, cursed the one who consumes usury, the one who pays it, its recorder, and its witnesses. Whoever leaves something for the sake of Allah, Allah will compensate them with something better and greater, and Allah will provide relief after hardship. Needing a place to live does not permit taking a loan with interest. And Allah Knows Best.