What is the ruling on performing voluntary prayers during official working hours?

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

There is no harm if  there was break time during official working hours, but if not then it is impermissible. And Allah Knows Best.

Is it incumbent on a husband to pay the alimony of his wife if she had left him and stayed at her parent`s without his permission?

When a wife leaves her house, and stays at her parent`s without asking her husband, she is considered a Nashiz (wife who refuses to abide by her husband’s orders), and so she doesn`t deserve an alimony.

What should one who forgot to offer the second Sajda during congregational prayer, and remembered it before the end of the prayer do?

All perfect praise be to Allah, The Lord of The Worlds.                                                                                                                                                          Whoever forgets a pillar of prayer while in congregation must follow the lead of the Imam, and after the latter offers Tasleem, he should offer one Rakah, then perform Sujood As-Sahw (prostration of forgetfulness). And Allah Knows Best.

Is it permissible for a wife to leave her house without asking her husband?

It is impermissible for the wife to leave her house without asking her husband except for a sound reason.