Who is the one deemed "able" (al-qādir), for whom the uḍḥiyah (sacrificial offering) is sunnah?
All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah ﷺ.
The one who is deemed "able" (al-qādir) [to perform the uḍḥiyah, i.e., the sacrifice] is someone who possesses wealth sufficient to acquire the sacrificial animal, over and above what is needed to cover his own living expenses and the living expenses of his wife and children — or of anyone else whose financial support (nafaqah) is legally obligatory upon him — for the day of ‘Eid and the three days of Tashrīq [that follow it], since this is the designated time period for the sacrifice. And Allah, the Exalted, knows best.
What is the ruling on istinja' after urination, and is it done with water and soap or with water only?
Istinja' from urine and stool is obligatory. It is permissible to perform istinja' with water alone, or with toilet paper alone. The best way is with toilet paper then water. If one wishes to suffice with one of them, then water is better. And Allah the Almighty knows best.
What is the ruling on performing voluntary (nafl) prayer during the time of istiwā' (the sun's zenith) on the day of Jumu‘ah (Friday), and is this ruling specific to someone who has come to the mosque to perform the Jumu‘ah prayer?
All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah ﷺ.
It is permissible to perform voluntary (nafl) prayer during the time of istiwā' (the sun's zenith, i.e., its highest point before noon) on the day of Jumu‘ah (Friday) — whether for someone who is attending the Friday prayer or someone who is not, and whether they are among the people obligated to attend it or not.
It is stated in Mughnī al-Muḥtāj (Vol.1/P.310): "The more correct position (al-aṣaḥḥ) is that prayer is permissible during this time without restriction — regardless of whether the person will be attending the Jumu‘ah prayer or not." And Allah, the Exalted, knows best.
Is a person who forgot to make up missed fasts until the next Ramadan began required to pay expiatory compensation (Fidyah) alongside the makeup fast (Qada')?
All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah.
Whoever forgets to make up missed fasts of Ramadan (Qada') until the following Ramadan begins incurs no sin, pursuant to the statement of the Messenger of Allah (peace and blessings be upon him): "Indeed, Allah has pardoned for my nation their mistakes, their forgetfulness, and that which they are coerced into doing." [Narrated by Ibn Majah, and Al-Bayhaqi in Al-Sunan Al-Kubra]
Furthermore, such an individual is not obligated to pay a expiatory compensation (Fidyah). It is stated in Bushra Al-Karim (p. 580)—one of the authoritative Shafi'i jurisprudential texts: "As for delaying it [the makeup fasts] due to a valid excuse—such as travel, breastfeeding, forgetfulness, or ignorance of the prohibition of delay (even for one living among scholars)—no Fidyah is required. This is because delaying performance during the actual month is permissible due to these excuses; hence, delaying makeup fasts is even more permissible, even if it continues for years."
And Allah Almighty knows best.