[quote=@Mistiel]Mark Jackson obviously stayed behind - presumably for one other mission-related reason (like guarding the clear codex of places so that he could monitor its use and protect the rest of the galaxy from humanity coming through to conquer other worlds - if not solely due to falling in love with the woman who ended up getting captured upon the earthlings' return.[/quote]
Daniel Jackson. Not Mark. And he stayed behind to bury the Stargate which would protect Earth. Because they thought that was the only stargate there was. It wasn't until later that he discovered the other gate addresses. Which is why he uncovered his gate. He was actually going to start trying to dial up another gate when Jack had dialed him.

[quote=@Mistiel](ooooh! wondering if SG1 explains the Egyptian concept of the Duat levels at all in a biological/astronomical way)
[/quote]I wouldn't count on it. Most the the mythology (not just Egyptian) is explored mostly superficially. Though if that is something of interest for the players we can do something a little more accurate and a little more in-depth.