"Adam and Eve were raw vegans and lived to be past 900 years old. Flesh eating wasn't introduced until AFTER the flood and the lifespan quickly dropped after then."
Animals were killed immediately after Adam and Eve's fall. The first were killed by God himself to "properly" clothe Adam and Eve.
The next evidence is Adam and Eve's children, Cain and Abel. Cain was a farmer and Abel was a husbandman. This first human family was to sacrifice animals to God for their sins. Most know of the story to follow. The vegetables that Cain offered were rejected by God whereas the animal sacrifice that Cain gave was accepted.
Cain raised animals for food and clothing (not just wool, but the skin as well).
I believe you will have a very difficult time proving from the scriptures your statement above.
As to the 900 years, I do believe that people lived a lot longer years ago (before the flood). The Bible tells us that there was a vapor canopy around the earth, not unlike some of our other planets.
This canopy basically created a hyperbaric greenhouse (as we've seen wooley mammoths instantly frozen solid in anarctic zones with tropical vegitation in their stomachs.
Also the vapor canopy would block UV light which alters our DNA. This is why the mandate to not marry one's sibling does not come until after the flood. I always wondered who Adam and Eve's children married until I understood this.
So the pre-flood was a much more life-supporting environment than the one we know today.