KSol-Is it your assumption that Noah and his family and their predecessors, prior to the Flood were carnivores? (Gen1:29) You emphasized "AFTER"..
But, it was only after the Flood that Noah and his family, then all of mankind to follow, were permitted to use animals as a food source ( Gen 9:3) And so why would a Divine command have been necessary prior to them being permitted to eat flesh in the first place. So, your point regarding Adam, Abel blood transfusion seems flawed.
The Bible doesn't explicitly mention whether they were strictly vegetarians, although it DOES mention that they were given fruits of the field to eat. However, a strict legalistic analysis of Genesis shows that there was no PROHIBITION against eating animals before the Flood, with or without their blood, so the Bible itself is mute on the subject.
Granted, there's no doubt God gave His explicit permission to eat animals in Genesis 9 (with the condition that they be properly bled first), but there was no prohibition against the practice BEFORE the Flood. Theologians (and lawyers) will tell you that any action NOT explicitly prohibited is not a violation of God's will, AKA a sin, but a conscience matter, at best.
Similarly, there was no prohibition against murder/manslaughter before the Flood, either: YHWH only demanded an accounting for spilled blood, animal and man, AFTER the Flood. Hence, Cain's murder of Abel was not a sin, and perhaps explains why YHWH let him off without demanding his own life in exchange for the blood he spilled. Remember: murder was only codifed as a sin AFTER the Flood, too, and hence YHWH was presumably patching the "evil thoughts in the hearts of man" defect of Genesis 3 fame with a legalistic fix, wiping the slate clean, but delegating the authority and giving the responsibility to humanity to create a judicial system for themselves.
(In fact, the whole idea of sacrifice as atonement for sin doesn't apply to Cain and Abel's story, as the only sin on record at that point was eating the forbidden fruit; their sacrifice could only be as a blessing offering.)
Of course, the Torah is a book of Hebraic law, so just like the story of Adam and Eve, the Flood is internal validation or a backstory of why civil and criminal law is needed (the Torah), and what happens when there are no laws (the evil that existed before the Flood).
That's a functional legalistic analysis of Genesis and the Torah, similar to that used by cultural anthropologists who look at the motivations and benefits served by a particular belief to the society or culture which adopts it....