"sin is not imputed when there is no law" refers to the age of grace (Which Romans is a large expounding of). If no one was imputed with sin before Moses, then why would they suffer any penalty of "death" (whichever form it may take) in the first place?
Romans 5:12 says "Wherefore, as by one man sin entered into the world, and death by sin; and
so death passed upon all men, for that all have sinned:"
It says "death passed" it doesn't say "sin passed." Death is inherited, sin is not inherited.
Ezekiel 18:20 says "The soul that sinneth, it shall die. The son shall not bear the iniquity of the father, neither shall the father bear the iniquity of the son: the righteousness of the righteous shall be upon him, and the wickedness of the wicked shall be upon him." So sin is not passed, death is passed, or inherited. This is how Jesus was able to die though he was sinless. If sin was passed, Jesus would have inherited sin. This is why Jesus says "
John 8:24 I said therefore unto you, that ye shall die in your sins: for if ye believe not that I am [he], ye shall die in your sins." If you inherit sin and death comes as a judgement to that sin, then you have already been judged before you die. But the scripture says "
Hbr 9:27 And as it is appointed unto men once to die, but after this the judgment:" If our judgement comes after we die, then the death within us is not a result of inheriting sin and being judged, but rather it is the judgement comes after death. This is why Jesus was resurrected... he died having been innocent of sin.
You basically seem to be saying, they weren't sentenced to spiritual death, because they weren't charged with sin, but they were only sentenced to a physical death instead (because of Adam's sin). But death is used as one penalty, of Adam's sin; not two different deaths as two different penalties of two different commands transgressed.
Physical death entered the world as a result of Adam's sin and reigned over those who did not sin the same sin as Adam. I am saying they were not "sentenced" to physical death, they "inherited" it. Many people have only half a view of judgement - they understand only the negative, but not the positive. Judgement can also result in justification! If you are judged innocent after death, you get back your life. Because the Bible clearly says "death is passed" we can know that we inherit death. When we die, we are judged - are you righteous or unrighteous. Because "all have sinned and fallen short of the glory of God" we are all judged unrighteous. All but Christ who was "born under the law" but did not sin (as opposed to those between Adam and Moses who were born without the law, sinned, but their sin was not imputed because there was no law).
The full Law wasn't codified until Moses, but there was still a universal ("Noahide") law that came after the Fall, which included murder being wrong, for instance. They were held accountable for this, beginning with Cain. Hence, Paul says death reigning "even over them that had not sinned after the similitude of Adam's transgression".
Everyone was spiritually dead.
That's a good point because God cursed Cain for his murder, so I don't mind including murder with "do not eat from the tree" in the list of pre-mosaic law.
And this is apart of the "sin nature", which is what was passed; (not just "sin" by itself). Jesus did not receive the nature, because He was the Son of God.
Again, the scripture is clear that death was passed, not sin. Jesus himself tells us where the sin nature comes from:
Jhn 8:34 Jesus answered them, Verily, verily, I say unto you, Whosoever committeth sin is the servant of sin.
Those who commit sin are in bondage to sin.
Hbr 2:14 Forasmuch then as the children are partakers of flesh and blood, he also himself likewise took part of the same; that through death he might destroy him that had the power of death, that is, the devil;
15 And deliver them who through fear of death were all their lifetime subject to bondage.
Rom 8:15 For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father.
Once again, Jesus describes the sin nature very succinctly;
Jhn 3:19 And this is the condemnation, that light is come into the world, and men loved darkness rather than light, because their deeds were evil.
20 For every one that doeth evil hateth the light, neither cometh to the light, lest his deeds should be reproved.
21 But he that doeth truth cometh to the light, that his deeds may be made manifest, that they are wrought in God.
Yet again, we're jumping back and forth between man and animals. Man is the one charged with sin, animals are not.
Because there is no law governing animal behavior. Yet God himself used the skin of an animal to cover Adam and Eve when they were naked and had sinned, setting the precedent of atonement.
Also, "soulish life" is one thing, but no one here says that plants have souls. The point is, their process of physical death is largely the same as ours.
Death affected soulish life. Since animals have soulish life, they were affected by death when death entered the world.
Incidentally, after Jesus return doesn't the scripture say that the "wolf shall dwell with the lamb" indicating that by human reconciliation with God, there will also be animal reconciliation to life?
Isa 11:6 The wolf also shall dwell with the lamb, and the leopard shall lie down with the kid; and the calf and the young lion and the fatling together; and a little child shall lead them.
7 And the cow and the bear shall feed; their young ones shall lie down together: and the lion shall eat straw like the ox.