The answers to your questions are as follows:
1. Marriage means a legitimate contract by which a man is allowed to enjoy a woman in exchange for a specific compensation, and its effects are the realization of Ihsan, guardianship, inheritance, and the spread of mahram relationships, and it is not required to be permanent rationally or religiously; rationally, because the principle is to not set any condition, and determining a term for marriage does not contradict what requires it, as it is concluded by being certain of divorce, but rather by its intention, and whatever is permissible permanently is a fortiori permissible temporarily; and religiously, because of the apparent meaning of the words of God Almighty 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 «إستمتاع»; “enjoyment” in «متعة»; “marriage”, and according to the understanding of Ubayy ibn Ka‘b and Abdullah ibn Abbas that includes ﴿إِلی أَجَلٍ مُسَمًّی﴾; “for a fixed term,” and explicitly states temporary marriage, and this is the saying of the Ahl al-Bayt of the Prophet, peace and blessings of God be upon him and his family, and the saying of many of the Sahabah, such as Abdullah ibn Abbas, Abdullah ibn Mas‘ud, Imran ibn Husayn, Jabir ibn Abdullah, Abu Sa‘id al-Khudri,and Salamah ibn al-Akwa‘, but rather of all the Sahabah until the end of the caliphate of Umar ibn al-Khattab, and it has been attributed to many great followers of the Sahabah, such as Tawus, Atta‘, Sa‘id ibn Jubayr, and other jurists of Mecca.
However, doing it secretly is against the Prophet’s Sunnah; because the Prophet’s Sunnah in marriage is to announce it, even with taking one or two witnesses, to prevent evils such as accusation, not to conceal it, which is appropriate for illegitimate relationships, and therefore, the more publicly the marriage takes place, the better, although it is not invalid if it is done secretly due to an excuse or necessity, and God is the All-Forgiving, the Most Merciful.
2. The disagreement of Islamic sects is not between the obligation and prohibition of temporary marriage, but rather between its permissibility and prohibition; in this way that Shia scholars consider it permissible and Sunni scholars forbidden; because in the supposition of Sunni scholars, although temporary marriage was permissible in the early days of Islam according to the apparent meaning of the Book of God and the mutawatir Sunnah of the Prophet, peace and blessings of God be upon him and his family, it was later abrogated by His Excellency in the battle of Khaybar, the battle of Tabuk, the conquest of Mecca, or elsewhere, while according to the view of Allamah Mansoor Hashemi Khorasani, may God protect him, in the honorable book Return to Islam, the rule is that it is not possible to abrogate the Quran with the Sunnah, and the Quran is abrogated only by the Quran; as God Almighty has said: ﴿مَا نَنْسَخْ مِنْ آيَةٍ أَوْ نُنْسِهَا نَأْتِ بِخَيْرٍ مِنْهَا أَوْ مِثْلِهَا﴾; “Any verse that We abrogate or cause it to be forgotten, We bring one better than it or like it”; regardless of the fact that the principle is to not abrogate the ruling of God until its abrogation is proven, while the abrogation of this ruling is not proven and has faced the denial of the Ahl al-Bayt of the Prophet, peace and blessings of God be upon him and his family, and the denial of many of the Sahabah and their followers.
Yes, some Sunni scholars have claimed that the verse ﴿إِلَّا عَلَى أَزْوَاجِهِمْ﴾; “except with their wives,” and the verse of inheritance have abrogated this ruling, with this statement that the first verse has condemned sexual activities with non-wives, while a woman contracted in temporary marriage is not one of the wives, and the second verse has considered the wives to be entitled to inheritance, while a woman contracted in temporary marriage is not entitled to inheritance, but it is clear that such a statement is pure fallacy; because if temporary marriage is permissible, the married woman in it is considered one of the wives, and the verse of inheritance does not invalidate her marriage, but rather it proves her inheritance, and this is clearer than to remain hidden from anyone, except for one who is lost in fanaticism and istihsan, while fanaticism prevents cognition of the truth, and istihsan has no basis in Islam; as God has said: ﴿فَعَسَى أَنْ تَكْرَهُوا شَيْئًا وَيَجْعَلَ اللَّهُ فِيهِ خَيْرًا كَثِيرًا﴾; “So perhaps you dislike something in which God has put much good.”
3. If a Muslim, in order to avoid a forbidden deed, needs temporary marriage, and for temporary marriage, he needs the help of other Muslims, then it is obligatory for other Muslims to help him as much as they can; as God Almighty has said: ﴿وَتَعَاوَنُوا عَلَى الْبِرِّ وَالتَّقْوَى﴾; “And help one another in goodness and piety,”especially if he is one of the family, relatives, orphans, the needy, neighbors, friends, and travelers; as God Almighty has mentioned them and said: ﴿وَبِالْوَالِدَيْنِ إِحْسَانًا وَبِذِي الْقُرْبَى وَالْيَتَامَى وَالْمَسَاكِينِ وَالْجَارِ ذِي الْقُرْبَى وَالْجَارِ الْجُنُبِ وَالصَّاحِبِ بِالْجَنْبِ وَابْنِ السَّبِيلِ﴾; “And do good to parents, relatives, orphans, the needy, close neighbors, distant neighbors, the companion by your side, and the wayfarer.”
4. Permanent marriage is certainly better than temporary marriage and has more blessings for the individual and society, and therefore, refraining from it despite the ability, to suffice with temporary marriage, is considered the abandonment of the preferred and contrary to wisdom and generosity; especially considering that the purpose of legislating temporary marriage in Islam has not been to prevent Muslims from permanent marriage, but rather it is a way by which God has made things easier for those who are not able to get married permanently and fear that they will fall into hardship; as He has said: ﴿يُرِيدُ اللَّهُ أَنْ يُخَفِّفَ عَنْكُمْ ۚ وَخُلِقَ الْإِنْسَانُ ضَعِيفًا﴾; “God wants to lighten your burdens, and the human being has been created weak.”
5. Marriage of a virgin is permissible with the permission of her guardian, and there is no difference in that between permanent marriage and temporary marriage.
6. If a man has made a promise to his wife that he will refrain from temporary marriage, then he must fulfill his promise; as God Almighty has said: ﴿وَالْمُوفُونَ بِعَهْدِهِمْ إِذَا عَاهَدُوا﴾; “And those who fulfill their promises when they make one,”and said: ﴿وَالَّذِينَ هُمْ لِأَمَانَاتِهِمْ وَعَهْدِهِمْ رَاعُونَ﴾; “And those who faithfully observe their trusts and promises,”and said: ﴿بَلَى مَنْ أَوْفَى بِعَهْدِهِ وَاتَّقَى فَإِنَّ اللَّهَ يُحِبُّ الْمُتَّقِينَ﴾; “Yes, one who fulfills his promise and is pious, God loves the pious,” unless he has no choice and is forced to do it to protect himself from committing a forbidden deed, in which case God is the All-Forgiving, the Most Merciful.
7. If young people will be polluted with forbidden deeds if they do not marry, then it is obligatory for them to marry in any way possible, even by borrowing, and it is obligatory for their family, relatives, neighbors, and friends to help them with it, and if they do not find a way to permanent or temporary marriage, then it is obligatory for them to be pious until God makes a way out for them; as He has said: ﴿وَمَنْ يَتَّقِ اللَّهَ يَجْعَلْ لَهُ مَخْرَجًا وَيَرْزُقْهُ مِنْ حَيْثُ لَا يَحْتَسِبُ﴾; “And whoever is pious, God will make a way out for him, and provide for him from where he does not expect,”and their piety is obtained from patience and prayer; as He has said: ﴿يَا أَيُّهَا الَّذِينَ آمَنُوا اسْتَعِينُوا بِالصَّبْرِ وَالصَّلَاةِ ۚ إِنَّ اللَّهَ مَعَ الصَّابِرِينَ﴾; “O you who believed! Seek help through patience and prayer; because God is with the patient,” and one of the greatest examples of patience is to preserve fasting that extinguishes the fire of lust and is considered a characteristic of the chaste; as God Almighty has said: ﴿وَالصَّائِمِينَ وَالصَّائِمَاتِ وَالْحَافِظِينَ فُرُوجَهُمْ وَالْحَافِظَاتِ﴾; “And men and women who fast, and men and women who guard their chastity.” Furthermore, it is obligatory for them to rise to prepare the ground for the advent of the Mahdi; because all their problems, such as the difficulty of marriage, are the consequences of the non-advent of the Mahdi who implements the rulings of Islam and revives the Sunnah of the Prophet, peace and blessings of God be upon him and his family, and therefore, those of them who do not rise to prepare the ground for the advent of the Mahdi are at fault for the difficulty of marriage for themselves and have no excuse for falling into a forbidden deed if they fall into it.
As a result, there is nothing wrong with temporary marriage with observing all the rulings of permanent marriage, such as the guardian, witnesses, the contract, the marriage portion, and iddah, but it is clear that it is not permissible to be extravagant with it, use it as a pretext for debauchery or wrongdoing against women, or transgress the hudud that God and His Prophet have set for it, and God has full knowledge of everything.