There's nothing wrong with changing the rules over time. The rules I set for my children change over time and in different circumstances, simply because different circumstances actually require different rules - or even because I just decided we'd do something different now, and I have the right to issue new instructions as I'm in charge. For instance, I might set bedtime as 7:30pm, then as a child gets older set bedtime at 9pm.
In this case, the circumstances most certainly changed. The biological reason to not marry siblings is because of mutations - damaged genetic material. Mutations accumulate over time, and we all have mutant genes - but most are not expressed. Let's say you are carrying a damaged gene that will harm eyesight. If you marry a woman who does not carry this gene, some of your children will get the damaged version from you, but all will get a good version from her. All your children will carry a copy of a good version of the gene, and all will have normal eyesight. But if you married your sister, and she also carried the same gene, some of your children will receive two copies of the damaged gene (one from you and one from her), and no good copies. Some of your children will be born blind.
Closely related people are more likely to carry the same mutations, distantly related people carry different ones. So marrying distantly related people results in less birth defects.
Now, Adam and Eve were created perfect - and had perfect genes. Their children would have had almost-perfect genes. There would have been zero actual risk in two siblings marrying at that stage, or for the first thousand years or so.
But as mutations accumulate over time, by the time of Moses it may have become risky to marry close relatives. Hence the rules against marrying sisters being introduced at that point. The situation had changed, and a new law was required.
Yes, the goal posts were moved. But what is the problem with that? In what way is it a logical fallacy? It's just a new rule for a new situation.
Likewise, Adam and Eve were only given plants to eat - but Noah was told he could eat animals. Why? I would assume that the pre-flood environment was better, and it was easier to get nutrition from plants. Post-flood the soil and atmosphere would both have been severely depleted of nutrients (for reasons I could explain but I won't get waylaid), so plant tissue may have been less nutrient dense, meaning that it became necessary for humans to have access to animals as food also. This is an assumption - the Bible does not give a reason - but it shows that it was not necessarily an arbitrary change.
God can change the rules over time, He is God and He can do what He likes. He doesn't change - but his instructions to us may. We cannot limit Him to our own extra-Biblical logic (and your assumption of Torah being given to Adam is extra-Biblical logic).
