The answers to your questions are as follows:
1. Nikah means a legal contract according to which a man is allowed to enjoy a woman with a determinate compensation, and its effects are the realization of marriage, guardianship, inheritance and the dissemination of mahramiyat, and its permanence is not a condition rationally or religiously; rationally because the principle is the lack of setting any condition and the determination of the period for marriage does not contradict its requirements, as it is concluded by divorce rather by its intention, and whatever is permissible forever will be permissible for a limited time with stronger reason, and religiously because of the appearance of God’s word that has said: ﴿فَمَا اسْتَمْتَعْتُمْ بِهِ مِنْهُنَّ فَآتُوهُنَّ أُجُورَهُنَّ فَرِيضَةً ۚ﴾; “So those of women whom you enjoy, pay their compensation as an obligation”, according to the manifestation of the word “enjoyment” in mut’ah and according to the interpretation of Abi ibn Ka’b and Abdullah ibn Abbas, which includes ﴿إِلی أَجَلٍ مُسَمًّی﴾ and is explicit in temporary marriage, and this is the word of the Ahl al-Bayt of the Prophet peace and blessings of God be upon him and his household, and word of many companions such as Abdullah ibn Abbas, Abdullah ibn Mas’ud, Imran ibn Husayn, Jabir ibn Abdullah, Abu Saeed Khudri and Salmah ibn al-Akwa’, rather all the honorable companions until before the end of the caliphate of Umar ibn al-Khattab, and it has been also attributed to many great followers such as Tawus, Atta, Saeed ibn Jubayr and other jurists of Mecca.
However, doing it secretly is against the tradition of the Prophet; because the tradition of the Prophet in marriage is to announce it even by taking one or two witnesses to prevent corruptions such as slander, not to conceal it which is suitable for illegitimate relations, and with this description, the more open the marriage, the better, although if it is done secretly due to an excuse or necessity, is not invalid, and God is the Merciful, the Compassionate.
2. The disagreement of Islamic sects is not between the obligation and forbiddennessof temporary marriage, but between its permissibility and forbiddenness; in this way that Shia scholars consider it permissible and Sunni scholars consider it forbidden; because to the supposition of Sunni scholars, although temporary marriage was permissible in the early days of Islam according to the appearance of the book of God and according to the mutawatir tradition of the Prophet peace and blessings of God be upon him and his household, later in the battle of Khybar or the battle of Tabuk or the conquest of Mecca or elsewhere, has been abrogated by his Excellency, whereas according to the original basis of Allamah Mansoor Hashemi Khorasani may God protect him, in the honorable book “Return to Islam”, abrogation of Quran by tradition is not possible, and Quran is abrogated only by Quran; as God has said: ﴿مَا نَنْسَخْ مِنْ آيَةٍ أَوْ نُنْسِهَا نَأْتِ بِخَيْرٍ مِنْهَا أَوْ مِثْلِهَا ۗ﴾; “Any verse which We abrogate or make memories forget it, a better verse or like that We bring”; regardless of the fact that the principle is the lack of abrogation of God’s law, until its abrogation be proven, whereas abrogation of this law is not proven and has been faced with the denial of the Ahl al-Bayt of the Prophet peace and blessings of God be upon him and his household, and the denial of many companions and followers.
Yes, some of Sunni scholars have considered the verse ﴿إِلَّا عَلَى أَزْوَاجِهِمْ﴾ and the verse of heritage the abrogators of this law, with this statement that the first verse has reproached sexual relation with other than wives, whereas the married woman in temporary marriage is not form the wives, and the second verse has considered the wives deserving of the heritage, whereas the married woman in temporary marriage does not deserve the heritage, but it is clear that such a statement is merely a fallacy; because if a temporary marriage is permissible, the married woman in it is considered from the wives, and the verse of heritage does not invalidate her marriage, rather proves her heritage, and this is clearer than that to remain hidden from anyone, except for the one who is wandering in conflict of prejudice and Istihsan, whereas prejudice prevents the cognition of the truth and Istihsan does not have any principle in Islam; as God has said: ﴿فَعَسَى أَنْ تَكْرَهُوا شَيْئًا وَيَجْعَلَ اللَّهُ فِيهِ خَيْرًا كَثِيرًا﴾; “So perhaps you dislike something and God has put a lot of good in it”!
3. If a Muslim needs temporary marriage to be free of Haram and the help of other Muslims for temporary marriage, it is obligatory on other Muslims to help him as much as they can; as God has said: ﴿وَتَعَاوَنُوا عَلَى الْبِرِّ وَالتَّقْوَى ۖ﴾; “and help each other in good and piety” and this is more emphasized about family, relatives, orphans, the poor, neighbors, friends and travelers; as God has mentioned them and has said: ﴿وَبِالْوَالِدَيْنِ إِحْسَانًا وَبِذِي الْقُرْبَى وَالْيَتَامَى وَالْمَسَاكِينِ وَالْجَارِ ذِي الْقُرْبَى وَالْجَارِ الْجُنُبِ وَالصَّاحِبِ بِالْجَنْبِ وَابْنِ السَّبِيلِ﴾; “and do good to the father and mother and relatives and orphans and the poor and relative neighbors and neighbors next door and companion friend and wayfarer in need”.
4. Certainly, permanent marriage is better than temporary marriage and has more blessings for the individual and society and with this description, abstaining from it despite the capability, to confine oneself to temporary marriage, is considered abandonment of the preferred and is contrary to the wisdom and virtue; especially considering that the purpose of legislating temporary marriage in Islam has not been to prevent Muslims from permanent marriage, rather has been a discount from God for those of them who are not able to have a permanent marriage and are afraid of falling in hardship; as He has said: ﴿يُرِيدُ اللَّهُ أَنْ يُخَفِّفَ عَنْكُمْ ۚ وَخُلِقَ الْإِنْسَانُ ضَعِيفًا﴾; “God wills to give you discount, and human was created weak”.
5. Marriage of a virgin is permissible with the permission of her guardian, and in this regard there is no difference between permanent and temporary.
6. If a man has made a promise of abstaining from temporary marriage to his wife, it is obligatory on him to fulfill his promise; as God has said: ﴿وَالْمُوفُونَ بِعَهْدِهِمْ إِذَا عَاهَدُوا ۖ﴾; “those who fulfill their promises when they promise” and has said: ﴿وَالَّذِينَ هُمْ لِأَمَانَاتِهِمْ وَعَهْدِهِمْ رَاعُونَ﴾; “those who observe their trusts and promises” and has said: ﴿بَلَى مَنْ أَوْفَى بِعَهْدِهِ وَاتَّقَى فَإِنَّ اللَّهَ يُحِبُّ الْمُتَّقِينَ﴾; “Yes, the one who fulfills his promise and acts piety, God loves the pious”, unless he has to do it in order to stay safe from the forbidden, in which case God is Forgiving the Merciful.
7. It is obligatory for the youths who become polluted with Haram if they do not marry, to marry in any way possible, although by borrowing, and it is obligatory on their family, relatives, neighbors and friends to help them in this matter, and if there is no way for them to permanent or temporary marriage, it is obligatory for them to act piety so that God opens a way for them; as he said: ﴿وَمَنْ يَتَّقِ اللَّهَ يَجْعَلْ لَهُ مَخْرَجًا وَيَرْزُقْهُ مِنْ حَيْثُ لَا يَحْتَسِبُ ۚ﴾; “And anyone fears God, He puts a way out for him, and He provides him from where he does not think of” and their piety depend on patience and prayer; as he said: ﴿يَا أَيُّهَا الَّذِينَ آمَنُوا اسْتَعِينُوا بِالصَّبْرِ وَالصَّلَاةِ ۚ إِنَّ اللَّهَ مَعَ الصَّابِرِينَ﴾; “O you who believed! Seek help from patience and prayer; because God is with patient people” and one of the greatest examples of patience is to observe fasting, which extinguishes the fire of lust and is the characteristic of the pure; as God has said: ﴿وَالصَّائِمِينَ وَالصَّائِمَاتِ وَالْحَافِظِينَ فُرُوجَهُمْ وَالْحَافِظَاتِ﴾; “and the men and women who fast, and the men and women who guard their chastity”. Moreover, it is obligatory on them to rise up to prepare the grounds for the advent of Mahdi; because all their problems, such as the difficulty of marriage, are the consequences of the absence of Mahdi for them, who implements the laws of Islam and revives the tradition of the Prophet peace and blessings of God be upon him and his household, and with this description, those of them who do not rise up to prepare the grounds for the advent of Mahdi are at fault for the difficulty of marriage for themselves and have no excuse for being in Haram - whenever they be in it.
The result is that temporary marriage is not a problem with observing all the laws of permanent marriage, such as guardian, witness, agreement, marriage portion and iddah, but it is clear that it must not become extravagant and become a cover for immorality or a means of oppression against women and transgress the limits which God and His Prophet have ordained for it, and God is Aware of everything.