Did Adam needed to understand the concept of death? Say if God told him to not eat of the fruit or else I will hit you with a cane. And Adam, just so that you will understand what it means to be hit with a cane, turn around. When we tell a small child not to play with fire, whe tell them that you might get hurt. We don't put his hand in the fire so that he will understand. So why did Adam need to understand death? I think God made it pretty clear to him that not eating is the important part.
And then of what death was He talking. Spiritual or physical. If spiritual then how could God explain that to him? Even physical death would have been impossible to explain, actually impossible (would mean death excisted vefore the fall).
All animals had to be herbivores before the fall, the same as Adam and Eve. Eating meat and death only started after the Fall with God actually the first one to kill something.
About microbes, well things where very different then. A lot of them were not needed, because they are often used in breaking down dead bodies (which none yet around). Flies live 2 days, but there most probably wasn't any flies around - result of the fall.
So I think God gave Adam a command and expected him to keep it. For this reason I think they might have even lived in the garden a lot longer than 7 days or years (most probably a maximum of 70 or so by the time their first children were born).