Right, I now have both MN and the expansion, and am mightily confused as to how far the two "overlap". The comments in the expansion rules seem to suggest that the two don't overlap at all, which isn't terribly helpful.
My questions are:
* Is it worth playing a four player game with a six player setup?
* Can I use the gods/monsters rules and the new heroes/wonders in a four or five player game on the original map?
* If my playing the original by the original rules, is it worth importing the "Greek Defence" rules from the expansion? Those new heroes/wonders from the expansion that are relevant?
* For those cards from the original which are replaced in the expansion should I be using the replacement versions if I'm playing solely by the original rules? How about the replacement setup sheets?
* Should I try playing the original as a stand-alone set before I import anything at all from the expansion?
* Is it worth playing a four player game with a six player setup?
Never tried it. I usually leave out the expansion map with 4 players. Atlantis is highly mobile though so it could work. Not sure which powers I'd leave out but I think this is recommended somewhere in the rulebook.
* Can I use the gods/monsters rules and the new heroes/wonders in a four or five player game on the original map?
Yes, that's how we do it. Just leave out pottery.
* If my playing the original by the original rules, is it worth importing the "Greek Defence" rules from the expansion? Those new heroes/wonders from the expansion that are relevant?
No, either play the original game, or just go for it and play the expansion game. The expansion rulebook fully replaces the original so it's not like you need to really cross reference anything.
* For those cards from the original which are replaced in the expansion should I be using the replacement versions if I'm playing solely by the original rules? How about the replacement setup sheets?
No, I wouldn't mix and match. Especially if you're a new player.
* Should I try playing the original as a stand-alone set before I import anything at all from the expansion?
No, the expansion really is not much more complicated. Just go ahead and play the expansion rules. Forget the original rules.