Hi Kantabricus!
1. I have downloaded the demo and played a little, however I do not find the tutorial to be too enlightning, there is just a small comment at the start of the scenario and that is it.
There should be 3 tutorial scenarios. In these scenarios the tutorial text boxes give you instructions what to do. There is a little arrow in each box. Click on it to switch to the next page.
2. I have tried to find the manual ... no luck ... does it come with the game when you purchase it?
You can find the newest manual here:
http://www.ageod-forum.com/forumdisplay.php?f=1653. Is it realy an easy game to play (even though it may be hard to master).
Well, this depends on your experience with strategy games.

You will need some time to learn the game mechanics but it is not overly complicated imho.
The least complex of our games is
Birth of America 2 - Wars in America. If you are interested in this time period and theater too, I would recommend to try this game first.

4. Is it possible to play online?
Yes, this is possible via an tool provided with the game (You connect directly with your opponent. But there is no official AGEOD server with a list of potential players.). Most gamers do multiplayer via PBEM however.
Cheers
Norbert