Is it valid to perform the obligatory prayer while sitting?
Praise be to Allah, and peace and blessings be upon our Master, the Messenger of Allah.
Standing (Al-Qiyam) while having the ability to do so is one of the pillars (Arkan) of the prayer; the obligatory (Faridah) prayer is not valid without it. However, whoever is unable to stand may pray sitting down. As for voluntary (Nafilah) prayers, it is permissible to pray them sitting even if one is able to stand, but the one sitting receives half the reward of the one who prays standing. And Allah the Exalted knows best.
What is the ruling on performing the Witr prayer as a single rak`ah?
Praise be to Allah, and peace and blessings be upon our Master, the Messenger of Allah.
It is permissible to perform the Witr prayer as a single unit (rak‘ah). It was narrated from Ibn ‘Umar that a man asked the Messenger of Allah ﷺ about the night prayer, and the Messenger of Allah ﷺ replied: 'The night prayer is offered two by two (mathna mathna). If one of you fears the approach of dawn, let him pray a single rak‘ah to make what he has prayed odd-numbered (Witr) for him.' (Related by al-Bukhari & Muslim)). However, limiting the prayer to only one rak‘ah is considered 'contrary to the preferred way' (Khilaf al-Awla).
It is stated in Al-Minhaj al-Qawim Sharh al-Muqaddimah al-Hadramiyyah (p. 137): 'The minimum of Witr is one rak‘ah, but limiting it to that is contrary to what is best.'
The most complete form of Witr is eleven units, while the minimum level of 'perfection' is three units. It is stated in ‘Umdat al-Salik (p. 60): 'The minimum of Witr is one rak‘ah, and its maximum is eleven, performing the taslim (salutation) after every two units. The lowest level of perfection is three units with two separate taslims (meaning 2+1).' And Allah the Exalted knows best.
Should missed Sunnah prayer be made up?
Making up missed voluntary acts of worship is from Sunnah.
What is the ruling on public street water if it gets on a person's clothes or body?
The default ruling is that public street water is pure (tahir). If one is certain of its impurity (najasa), then a small amount that gets on the lower part of a person's clothes or body is overlooked (excused). And Allah the Almighty knows best.
