Where are you getting the idea that Abel understood his sacrifices to be substitutionary, or that Cains was rejected because it wasn't bloody? Cains was told by God why his sacrifice wasn't accepted, and it was because of who Cain was not anything to do with the sacrifice itself.
"“Why are you angry? And why has your countenance fallen? 7 If you do well, [
e]will not
your countenance be lifted up? And if you do not do well, sin is crouching at the door; and its desire is for you, but you must master it.”
Hebrews tells us why Abels sacrifice in other words, and that is this:
By faith Abel brought God a better offering than Cain did. By faith he was commended as righteous, when God spoke well of his offerings. And by faith Abel still speaks, even though he is dead.
So it is faith, and not the offering itself, that marked the difference between Cain and Abel in their sacrifices.
And if we look to what Jesus said about sacrifices we have the following:
If only you had known the meaning of 'I desire mercy, not sacrifice,' you would not have condemned the innocent.
Jesus surely served as a legal substitute, but if you think God is beholden to lavishing out punishment rather than showing mercy and compassion I'm afraid you missed the heart of our Creator.