The flaw in your second scenario is that a firm purpose of amendment would not include marrying someone else. It might involve begging the first wife to take him back, and if not, then living in a silent witness to his word which he gave when he was married to be faithful to her alone. You seem to think that word just dissolves away into nothingness, but it is a vow taken before God and witnesses. If he cannot live with her, he should not remarry as long as she lives. Or his word is meaningless. It loses the sign value then of Christ and His spouse the Church, where in your second scenario Christ takes up with another woman and leaves the Church to fend for herself. Marriage lasts a lifetime because Christ and His Church are married and will not divorce. That bit of ecclesiology informs the sacrametality of marriage. Dissolving away the lifetime commitment of marriage makes people wonder about how committed Jesus is to His Church.
Well, as already stated, St. Epiphanius, Bishop of Cyprus (died 403AD) provided written testimony that if a man becomes separated from his first wife for a valid reason, that "if he take another wife" he is not excluded from the Church or the life. The synod of Orthodox Bishops state that this is the Orthodox way.
As far as your perspective on marriage as only a temporary covenant pertaining mostly to the concerns of life in this world that is passing away, we have to wonder how it can effectively really be the sacred heavenly mystery that is somehow connected to the eternal marriage bond that exists between Christ and the Church, if our marital bond in our human marriages simply dissolves away upon the death of one of the people who of whom God says "they are no longer two, but one flesh". Tell us, will we also no longer consume the body and blood of God when we are become "as the angels" feasting at the heavenly banquet, in the same way that we shall no longer be of one flesh with our own husbands or wives with Whom God permanently joined us? We thought that Sacraments always pertained to eternal Life and have no meaning outside of that context.
We also wonder if the majority of Christendom does not suffer from the same need for being taught by Christ about the purpose of the law that the Jews whom Christ was addressing had, when He came to give the more perfect revelation of God to us. Roman Catholic and Protestant perspectives on the marital covenant do not currently seem to go beyond what was already the mind of the Jews who were coming to Christ with their questions about marriage and divorce, as being nothing more than a legal arrangement with value only in this temporal world and in preparation for Life in God's Kingdom. But Christ reveals that marriage is far more than this in the eyes of God, especially when He says "But I tell you that anyone who looks at a woman to lust after her has already committed adultery with her in his heart." (Matthew 5:28)
They all thought of themselves as obedient to God's command forbidding adultery as long as they had the appearance of those abiding by the letter of the law, Yet Christ spoke to them about this in a way that shows that they did not have the slightest spiritual understanding of the eternal implications of marriage, adultery, or divorce, because the were carnal and base, being greatly under the influence of their sin-hardened and faithless hearts. For until He said it, they were not even aware that they were indeed adulterers themselves, simply by virtue of looking upon someone besides their own spouse and lusting after them. How many young Roman Catholic married men and women would you guess have done this at one time or another, and continue to do it very automatically, thereby closing the door of their hearts to God by the power of this sin? How many Roman Catholic clergymen see many objects of their sexual desire and lust after that which they see - and then act on it? How many Roman Catholic bishops, when they receive accusations against such clergymen react to those accusations in strict accordance with the Apostolic laws by deposing them, rather than just moving them to another region to serve as clergy and receive Communion at the altar at some other geographical location where their past predatory sexual activities are not known about (refer to Canon 25 & 26 of the Apostolic Canons)? And yet for a long time such corrupt priests and bishops continue to be in the Church, presiding at the altar of God and consuming the Eucharistic gifts, at least until somebody finally does something about it.
But if one has suffered being divorced from their spouse, for reasons of sexual infidelity that has undermined and destroyed the bond that God made (because people can do this by the wrongful exercise of their freewill), and having been cheated out of being married they take another spouse, because of their inability to maintain celibacy, These are excluded from the Church and the life.
But if they can receive an annulment of their first marriage, which is where a Roman Catholic bishop determines that God did not really join those two into one flesh, so they were never really married, just fornicators because they thought they were married, then they may be married to someone new and not excluded from the Church and the life. This idea is very alien to Orthodoxy and to the divine word, because even though a bishop has authority to forgive or retain sins, They, as men, do not however have the authority to determine if the joining of the two persons into one flesh in marriage had been in accordance with God's will, or not. They may not make null what God has joined together. Sin can kill the covenant, but not the decrees of men. Men, given the authority to do so by God, can declare the sin to be forgiven, on the basis of what is best for people, with their eternal salvation being the greatest concern, and can also allow people who have become married to someone else after being divorced by their first spouse, to be included in the Church and the Life in Christ. The divine word, as St. Epiphanius has written and as the Holy Synod of Bishops in the Church have decreed, does not exclude the remarried from the Church and the full life of a member of the Church, except for possible periods of penance whenever spiritually expedient and beneficial.