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.
Must a person refrain from eating for the rest of the day if they break a fast of a vow (nadhr) or a make up fast (qada)?
Whoever observes a vowed fast (nadhr) or a makeup fast (qada) is prohibited from breaking it without a valid excuse. If they break it without a legitimate reason, they are sinful.
However, they are not required to refrain from eating for the rest of the day, because such restraint is only required out of respect for the month of Ramadan, not for other types of fasting.
How should I deal with the whispers of the devil with regard to ablution and purity?
Whispers in ablution are from the devil, and so you should take refuge in Allah from the devil`s evil suggestions that aim to distort the religion. Thus, once an organ is completely pure, don`t question its purity since having whispers doesn`t mean that the purity is invalid as it has already been established.
What is the ruling on someone who combines two prayers as an early combination (jam‘ taqdīm), and then arrives at his place of residence during the time of the second prayer — is he obligated to repeat it?
All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah ﷺ.
Whoever combines Ẓuhr and ‘Aṣr while traveling — that is, before he has reached the built-up boundaries of his hometown [upon returning] — his combination (jam‘) of the two prayers remains valid, even if he arrives at his place of residence before the time for the second prayer has actually begun. This is because the combination was already validly completed with its proper conditions fulfilled, and he is not obligated to repeat the second prayer [upon arrival].
A traveler is permitted to make use of the concessions of shortening (qaṣr) and combining (jam‘) prayers once the distance of travel reaches (81 km) or more, provided that his journey is a permissible one, and that he begins shortening and combining only after actually setting out on the journey and passing beyond the built-up limits of the city in which he resides. And Allah, the Exalted, knows best.