I think there’s a bit to be said contrary to both your lines of logic.
In Romans 5:12, a Scripture I and many others have quoted frequently for support, Paul says “death SPREAD” to all.
Interesting word that ‘passed’ or ‘spread’ as the word Paul used, “dielthen” has as a meaning to travel through a country or to passage from one place to another.
Hence Paul is speaking a progressive effect rather like a damaged printing plate continually making copies of its flaws and not something done to each copy individually.
In vs. 14 Paul goes on to say that even those who did NOT sin in the way Adam had were still subject to king death down to the time of Moses.
But then, What IS sin? Paul used the wore “hamartia” which has a literal meaning of “missing” a goal or marked aimed for and was used to describe actions too that failed to meet ethical standards.
“death spread to all men” because none could hit the mark of perfection as Jesus did.
Adam and Eve evidently understood what the consequences of their actions would be for their off-spring, they covered their reproductive parts realizing they could only pass on the death that had entered through Adam.
They had, in effect, killed everyone by their actions just as much as an airline pilot who fails to heed warnings and instructions and takes his charges into the ground.
“Sin is not passed on through “flesh”. Adam we agree was made, “flesh” without sin. Sin nature came about when he “sinned” i.e. performed a sinful ACTION. No genetic changes happened in Genesis 3”
I disagree. Peoples bodies began to wear out after a time, for Adam a very long time, and that time became less and less as that original vigor of perfection was lost.
Noah was born just decades after Adam died yet look at he drop in human life spans, the errors in the original copy were not being corrected but are increasing like a damaged printing plate making copies that are likewise damaged. It wasn’t that their sin was greater but that the flesh was weakening. Sin changed their fleshly inheritance too.
Further as you said,
“Sin nature is not “fleshy” — it’s not just that Adam’s body, genetics was given the sin-nature, we humans are more than just water and flesh. Similarly, even demons are sinners, but they have no “flesh”. “
In deed we are more than “just flesh and water” which would explain why no amount of genetic tinkering or good clean living can overcome the effects of our inheritance of imperfection.
“..death spread to all men because they had all sinned”.
The example of those angels that made themselves ‘demons” by sinning brought these questions to my mind:
What effect did their sin have upon their heavenly bodies?
The Bible is silent.
Can Christ sacrifice act as a ransom for these ‘demons’.
The Bible indicates No.
Since these non-fleshly creatures do not reproduce can they claim they inherited imperfection?
Again, the Bible indicates No.
But they DO miss the mark, sin, and suffer the consequences.