What is the ruling of Islamic Law if the follower (maʾmūm) stands up for the third rakʿah out of forgetfulness while the imam has sat for the middle tashahhud?
All praise is due to Allah, and peace and blessings be upon our master the Messenger of Allah ﷺ.
If the imam sits for the tashahhud while the follower (maʾmūm), out of forgetfulness, stands up for the third rakʿah, he is obliged to return to his sitting position, since following the imam is obligatory. In this case, no prostration of forgetfulness (sujūd al-sahw) is required of him. If, however, he fails to return, his prayer is rendered invalid.
Shaykh al-Islām Imām Zakariyyā al-Anṣārī, may Allah have mercy upon him, states: "If the follower stands up alone out of forgetfulness, he is obliged to return, since following the imam is obligatory... If he does not return, his prayer is rendered invalid, due to his departure from what is obligatory." [Asnā al-Maṭālib,Vol.1/P.190] And Allah the Almighty knows best.
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.
What is the ruling on using the *subḥah* (prayer beads) to count during *tasbīḥ* and *dhikr*?
All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah.
It is recommended to use the ordinary or electronic subḥah (prayer beads or counter) to count tasbīḥ, dhikr, and istighfār, because it helps one to remember Allah. Many reports have been transmitted from the Companions and the Followers (tābiʿūn) showing that they continued to use strings of date pits as an aid to dhikr and tasbīḥ.
It is one of the lawful means of worship. Indeed, what is established from the Prophet (peace and blessings be upon him), namely that he counted tasbīḥ on his right hand, shows that it is permissible to count tasbīḥ with whatever helps a Muslim keep track of the number. There is therefore no basis for calling it an innovation (bidʿah), and we know of no jurist who said so.
Al-Ḥāfiẓ al-Suyūṭī (may Allah have mercy on him) wrote a dedicated treatise on the subject titled al-Minḥah fī al-Subḥah, printed within al-Ḥāwī li’l-Fatāwī (vol. 4–6).
Shaykh al-Islām Imam Ibn Ḥajar al-Haytamī (may Allah have mercy on him) was asked: Does the subḥah have a basis in the Sunnah? He replied: “Yes, and al-Ḥāfiẓ al-Suyūṭī wrote on this. Among the evidence is what is authentically reported from Ibn ʿUmar (may Allah be pleased with them both): ‘I saw the Prophet (peace and blessings be upon him) counting tasbīḥ on his hand.’ Ibn Abī Shaybah, Abū Dāwūd, and al-Tirmidhī also narrated: ‘Hold fast to tasbīḥ, tahlīl, and taqdīs, and do not be heedless, lest you forget the affirmation of Allah’s oneness (tawḥīd). Count them on the fingertips, for they will be questioned and made to speak.’ Counting tasbīḥ with pebbles, date pits, and a knotted string has also been reported from a group of the Companions and those after them. Some scholars held that counting on the fingertips is better than the subḥah, based on the ḥadīth of Ibn ʿUmar. Others distinguished, saying: if the one reciting is safe from miscounting, counting on the fingertips is better; otherwise the subḥah is better.” [al-Fatāwā al-Fiqhiyyah al-Kubrā, Vol.1/p.152]. And Allah Almighty knows best.
Can someone who begins a voluntary fast break it?
It is preferable for someone who begins an act of worship not to break it.
Allah the Exalted has said {what means}: "and let not your [good] deeds come to nought!" [Muhammad/33].
However, if a person starts a voluntary fast (nafl) and needs to break it, they are going against what is preferable, but there is no sin upon them.