My friend you are right on this point, it doesn't make sense, fornication is sin because if breaks the sacred covenant of marriage, it is deception from satan that says that sin will be allowed in heaven, God is too pure for such things, God is not a liar, all sin will be punished in this life and the next, to dishonour God with such things that contradict His character and make double standards of His Holy nature is wrong and I'm glad you see this.
Also drunkeness is BONDAGE and sin, you come under the influence of a neurotoxin called alcohol and is damages your brain, your organs and makes you do things that you would not normally do because you cognative control over you body breaks down and you become open to demonic attack and violence and sexual sin and all manner of unclean-ness, this is NOT God's will, and God does not endorse alcoholism in Jannah.
Morality does not change between now and eternity, you are right!
Consider Adam, the first man, who was before Mohammed, before Abraham, Before Isa, Before Moses, He... Adam alked and talked with God in Eden the Paradise of God, this was before Sin, isn't THAT meant to be the real Muslim way? Isn't that how God FIRST designed humanity? It says in the Quran Surah 10:94, If thou wert in doubt as to what We have revealed unto thee, then ask those who have been reading the Book from before thee: the Truth hath indeed come to thee from thy Lord: so be in no wise of those in doubt.
The book before the Quran was the Bible, if you have questions the the Quran and Sunnah do not clarify in totality for you, then you are instructed to search the Torah and Tanach (Old Testament Bible), and Brit-Chadachar (New Testament Bible), I suggest you do this my friend.
The Quran says that when you are born you are innocent and pure until you sin, yet we know that any corruption in this world even sickness is a result of sin, when a Baby is born with birth defects and deformations, or stillborn, yet the Quran would say they who are newborn are pure as they have not sinned yet, why then are they born with a curse and corrupt? It is inconsistent with reality, If all children are pure in the sight of God, then they should not be born with a curse upon them, God who gives all life does not bring fourth corruption! What then, Do we still say that they are pure and clearly their physical state is not so? No, we look to the Bible and get the complete understanding. The Bible says that Sin entered the world through Adam and then like corrupt blueprint all products tha are made from it will too be corrupt, and Adam being corrupt after sin planted corrupt seed into a corrupt eve who gave birth to corrupt humanity, and the curse of sin is within all humanity by default, that's why people sin, because it's built in, not because we attain sin, what is already there it just coming out. We cannot change our own nature by ourselves or by any physical acts we do for religion, the inside nature of the man is the same, the only way to attain to Holiness and get back what was lost in the paradise of God before Sin is the change the inside nature of the man, the Ruhe the Spirit of the man, only God can do this, not us.
Again I say you need to read from the book before the Quran, Isa is the only man to not sin, the one who Christians call Jesus, learn about Isa in both the Quran AND the Bible, get a complete perspective my friend.