Some Rulings of Iddah

Summarized Fatawaa

What is the ruling on someone who perform their prayer after finishing it because they believe they missed a Rak'a, a prostration, or that they did not perform it correctly (thinking their prayer was invalid)?

If he was sure that his prayer is void then reperforming it is an obligation along with figuring out the reason of invalidity so long as this wasn't out of uncertainty. And Allah Knows Best. 
 

Is it allowed for a Muslim woman to wear nail polish?

Praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Nail polish is an adornment that a woman is allowed to wear for her husband, and not in the presence of non-Mahram men. However, during ablution water needs to reach the nails, so it isn`t possible to perform 'Wudu (ablution) with nail polish on. Therefore, it isn`t valid to make Ghusl Jnabah (Ghusl performed after sexual intercourse or ejaculation), Ghusl after menstruation and Ghusl after Nifas (blood of childbirth) while wearing nail polish. And Allah The Almighty Knows Best.

What is the ruling of Sharia on marrying the maternal niece of one`s wife?

All perfect praise be to Allah the Lord of The Worlds. May His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
One may not combine in marriage a woman and her paternal aunt, or a woman and her maternal aunt, as stated the Prophet (PBUH). And Allah The Almighty Knows Best.

Is a menstruating woman obliged to pray during menses, and should she make up missed prayers?

All perfect praise be to Allah, The Lord of The Worlds                                                                                                                                                                        A menstruating woman is prohibited from prayer during her menses, and she isn`t obliged to make up missed prayers. And Allah Knows Best.