What should a person, who doubts the validity of his Wudu, or over performs it, do?
One who doubts the validity of his/her Wudu while performing it should redo it until he/she is certain of having attained purity. But, if doubt was after having performed Wudu, then he/she should not pay attention to that as doubt after the end of an act of worship does not count. This is of course in case doubt was within the reasonable limits since once it goes beyond that, it becomes a whispering of the Shaytaan (devil) which he/she should ignore as the Wudu is valid.
What is the meaning of the Prophetic statement that a boy is held in pledge (murtahan) for his 'aqīqah?
All praise is due to Allah, and may peace and blessings be upon our Master, the Messenger of Allah.
The first interpretation: That if the boy dies in infancy without an 'aqīqah having been performed on his behalf, he will not intercede for his parents on the Day of Resurrection. This is the position of Imam Aḥmad ibn Ḥanbal, and Imam al-Khaṭṭābī concurred with him, stating: "The finest of what has been said regarding this matter is the position adopted by Aḥmad ibn Ḥanbal." — [Fatḥ al-Bārī by Ibn Ḥajar,{Vol.9/P.594]
The second interpretation: That the child is likened to a pledged object (marhūn) — one from which full benefit and enjoyment cannot be derived until it is redeemed. A blessing is only made complete upon the one blessed when they fulfil the obligation of gratitude (shukr), and the prescribed expression of gratitude for this particular blessing is what the Prophet ﷺ established as Sunnah — namely, the slaughtering of the 'aqīqah on behalf of the newborn as an act of thankfulness to Allah the Almighty and as a supplication for the wellbeing and safety of the child. This is the position of Mullā 'Alī al-Qārī. See: [Mirqāt al-Mafātīḥ Sharḥ Mishkāt al-Maṣābīḥ, Vol.7/P.2688]
And Allah Almighty knows best.
Is it permissible to trim the eyebrows if they become excessively long?
Trimming eyebrows is permissible if they become excessively long, yet; it is reprehensible. And Allah Knows Best.
What is the Islamic ruling on the aqiqa?
All praise is due to Allah, and may peace and blessings be upon our Master, the Messenger of Allah.
The 'aqīqah is a confirmed Sunnah (sunnah mu'akkadah). Two sheep are to be slaughtered for a newborn boy, and one sheep for a newborn girl. This is established by numerous Prophetic traditions, among them:
The narration of Samurah ibn Jundub, may Allah be pleased with him, who reported that the Messenger of Allah ﷺ said: "Every child is held in pledge for his 'aqīqah, which is slaughtered on his behalf on the seventh day, and he is named, and his head is shaved." — Narrated by al-Tirmidhī, who graded it as ḥasan ṣaḥīḥ.
And the narration of 'Ā'ishah, may Allah be pleased with her, who said: "The Messenger of Allah ﷺ commanded us to slaughter one sheep as 'aqīqah for a girl, and two sheep for a boy." — Narrated by Aḥmad and Ibn Mājah.
The imperative in these narrations is understood to denote recommendation rather than obligation, based on the ḥadīth of 'Amr ibn Shu'ayb, on the authority of his father, on the authority of his grandfather, who said: The Messenger of Allah ﷺ was asked about the 'aqīqah, whereupon he said: "Allah does not love 'uqūq" — as though he disliked the name itself — and then said: "Whoever has a child born to him and wishes to offer a sacrifice on their behalf, let them do so: two equivalent sheep for a boy, and one sheep for a girl." — Narrated by Aḥmad and Abū Dāwūd.
The legal inference drawn from this narration is that the Prophet ﷺ linked the slaughter to the wish and willingness of the individual, saying: "whoever wishes to offer a sacrifice... let them do so" — thereby indicating that the 'aqīqah is recommended (mustaḥabb) and not obligatory (wājib).
And Allah Almighty knows best.