The logical gymnastics are entertaining.
It's a simple formula from freshman logic courses "If A implies B and B equals C than A implies C"
If Mary is the Mother of Jesus and Jesus = God. Than Mary is the Mother of God.
The logic can be broken in one of two ways. Either Mary is not the Mother of Jesus. Or Jesus is not God. Arguments that seem to differentiate between Mother of Jesus and Mother of God imply that the speaker does not believe that Jesus is God (fitting my response to your first argument). Arguments like your second that speak of "natures" attempt to say that Mary was not, in fact, the mother of Jesus. That Jesus was 50% God and 50% Man when we know that it is not so. Such argument are based on human reasoning and opperate in reverse: "We know that Mary can't be very important because those darn Catholics like her so much, so she can't be mother of God"
So it is two challenges, one to show an effect on faith and morals, the other to show that any claims about Church authority (temporal) were profoundly influenced by the decretals.
Then both are already accomplished. The Church had no legit claim to land or title before the decretals nor to any "temporal" authority. The Decretals bought them believeability they did not have without them. No one doubts that these men were making claims prior to the advent of the decretals. But no one paid any real attention until the would be 'official' documented 'proof' popped up out of the blue. That temporal authority vested in the Church the ability to proscribe things they would not otherwise have been able to proscribe - period.
Proof, Havoc? I am not taking your word for it. Sorry. The challenge is not to provide an opinion, but to provide proof.
As to matters of faith and morals, Absent temporal power, the Popes never would have been able to foist the teachings and deeds of the inquisitions upon the world. Let's don't forget that 8 laws were contained in the decretals that specifically address the right of the church to do away with heretics (that's putting it nicely). This is both a matter of faith and morals and was made so by Papal Bull.
It has not been shown that Church policy on heretics is a matter of faith and morals.
But more explicitly, the "decrees of same" are forged decrees going straight to matters of faith and morals. Forged matters of doctrine by virtue of being decrees became law of the Church. Thus, it most certainly - in more ways than one, affected both faith and morals.
If you have some evidence, show it.
SD
I think the problem is trying to apply logic to something that is not logical. How can you reason about a mystery?