Does the comic make a reference as to when Lucifer fell? Even Biblical test aren't clear. The show mentions the flaming sword guarding Eden, what would it be guarding it from if not Lucifer, which clearly didn't work . A lot of fans are confused by a comment from Welling that Marcus is older than Lucifer. If your assumption of him and the "first" family is right then indeed he could be older than Lucifer. That is if Samael fell after
Yes. In the comics, Lucifer is 20 billion years old today. He places his fall when he was 10 billion years old.
The flaming sword is mentioned just as the protector of the tree of knowledge as far as I remember and not exactly of the garden as a whole.
As from what it was protecting it from? Well in the Bible I have no idea but in the comics, we have seen that God was created by external forces outside of his creation and that sometimes these forces make an appearance but as you can expect for the plot, they are never friendly towards Lucifer, the Creation or what’s within it.
Now the last part of your question has been asked before but I;l have to put the KR banner here just in case