Ok, so let me see if I understand this correctly.

So once I've started to GM a game with real players, I cannot return to the "Prepare Campaign" area to add new content.
This is because all changes are saved into the "active sessions". Am I thus able to load this "active session" as I would a "Prepare Campaign" session so that I can content?

This is the bit that I've found difficult to wrap my head around.
As it seems that if changes are made in Prepare Campaign and players load into that they are brand new characters again.
And if the save is loaded, the players have their characters (including loot & experience) but the additional content that was added while they weren't online is not there.

So from what you've said I can load the save from Active GM Session and add new content like scenes, monsters etc...?

Just trying to clarify it, as I don't want to start a campaign with friends just to find out I should have just finished the entire thing because now new changes are not seen.