Salat al-layl refers to additional prayers performed by Muslims at late night.
Of course you can.
Night prayers are Mustahab and you can decrease the number of the zikrs as well as the number of the prayers itself, i.e. if you can't perform all the 11 Rak'ats at least pray the last three or even the last Rak'ah.
Many hadiths have emphasized the importance of the night prayer. In his will to Imam 'Ali (a), the Prophet (s) emphasized the importance of the night prayer three times. And in a different saying, the Prophet (s) addressed all Muslims by: "I recommend you to say the night prayer, even one rak'a, since it prevents one from committing sins, quenches God's anger and extinguishes the fire on the Dooms Day".
It is so important that it is recommended to perform its qada' if one fails to perform it in its valid time and the person who fails to perform it is considered as a loser. It is recommended that the night prayer be performed even when one is tired or on a travel. Some hadiths from Sunni sources imply that the night prayer used to be obligatory in the early days of Islam, but it became recommended later.
According to the words of the Infallibles (a), committing sins and overeating cause one to lose his or her occasion to say the night prayers.
For more info see https://en.wikishia.net/view/Night_Prayer
Salatul Layl was practiced by the Prophet (SAWA) and Imam Ali (AS) and Lady Khadija from the beginning of Islam. Sincere believers used to do it as a recommended worship. We read in Quran (Verily, your Lord knows that you stand in praying at night, less than two thirds of the night or half of the night or a third of the night and also a number of believers with you) Sura Al-Muzzammil, verse 20. Sura Al-Muzzammil was from the very early Suras revealed on the Prophet (SAWA) in Makkah.
We can pray for non Muslim parents, relatives and friends who are still alive, to Allah (SWT) to guide them to The Right Path and enlighten their hearts and minds with the light of the Truth.
For those who passed away as non Muslims, we can pray to Allah (SWT) to deal with them with His Mercy.
We are not allowed to seek forgiveness for those who insisted and died on Shirk (polytheism) as we read in Quran, Sura Tawbah, verse 113: It is not proper for the Prophet and the believers to seek forgiveness for the Mushriks, even though they be of kin.
Yes. Recommended Prayers especially Salat al-Layl ( Night prayer) is highly recommended for every one including those who have unfulfilled Qadha prayers on them. No doubt, the reward of performing Qadha prayers is much more than the reward of recommended prayers.
You must perform every prayer within its time and never miss any obligatory prayer.
Yes we can repeat recommended prayer but one time is for us and other times with the intention of on behalf of others like parents, relatives and friends whether alive or dead.
Tahajjud time is during the night ( starting from either midnight or even after Isha) and ending with Fajr time. Tahajjud can be also performed after Fajr as Qadha.
Tahajjud is a great recommended worship for every night of the whole year. It is also called Salat Al-Layl or Namaz Shab. It is performed individually and not with congregation. Umar Ibn al-Khattab ordered during his government to pray Tahajjud with congregation which was then called Taraweeh.
Taraweeh is a Bid'ah which was never practiced by the Prophet Muhammad (SAWA) but was initiated by Omar Ibn al-Khattab in year 14 after Hijra.
Namaz Tahajjud can be performed either fully as 11 Rak'ats or shortly as three Rak'ats (2 Shaf' and one Witr) or in the shortest way which is just one Rak'at which is called Witr.
Yes, we can pray for our Sunni friends to be granted the light of following Ahlul Bayt (AS) who are the real source of the authentic Sunnah of the Prophet Muhammad (SAWA).
Many different explanations have been given regarding to the two terms shaf' and watr, as they have been used in the holy Quran (surah al-fajr, verse 3).
Al-Shaf' is usually translated as the 'even number', and al-watr is translated as odd number.
One meaning given is that all creation is made of pairs, and Almighty God being Singular.
Another meaning is the Shaf' prayer being a even number of units (i.e. two rak'ahs), and the Watr prayer being an odd number of units (i.e. one rak'ah).
May the Almighty grant us the tawfeeq of upholding such a spiritual and important practice of praying Salat al-Layl.