Is the son of the sister by breastfeeding considered a mahram in Islam, and is it permissible for a woman to go for Hajj with him?
Everything that is prohibited by lineage is also prohibited by breastfeeding. The son of the sister by breastfeeding is considered a mahram, just like the son of the sister by blood, and it is permissible for him to be her mahram for Hajj and Umrah. And Allah Knows Best.
I swear that I will leave the religion of Allah if I return to smoking" and then returned to smoking. What is the ruling?
Whoever conditions disbelief on something and intends to disbelieve if it happens, he becomes a disbeliever immediately. Therefore, you must pronounce the two testimonies of faith (Shahada) and reconcile with your wife if she is in her waiting period (Iddah). It is recommended to consult the Department of Fatwa for further guidance. And Allah Knows Best.
Is Zakah (obligatory charity) due on the Zakah money received by a poor person, and reached a Nissab (minimum amount liable for Zakah), and a whole lunar year had lapsed over having it in his possession?
Yes, the poor who possessed a Nissab for a whole lunar year is obliged to pay the Zakah due on that money even if it was given to him as a Zakah money in the first place. And Allah Knows Best.
What is the ruling on having a second congregational prayer in the mosque?
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 It is disliked to have a second congregational prayer in the mosque which isn`t visited by travelers like the neighborhood`s mosque, and which has an appointed Imam, save with the latter`s permission. Nevertheless, if the mosque is visited by travelers, it is permissible to have a second and a third congregational prayer, whether it has an appointed Imam or not, or whether he permits that or not. And Allah Knows Best.