There was a beta for Conquests at least where some of the early modders here participated. I'm not sure about the other versions. Yes to unused content as well. There is artwork for Charles V of Austria and the Hussar unique unit, which do not appear in any official scenarios. There are also "Easter egg" units for Elvis, Santa, and a zombie(?) which only appear in place of King units if you play regicide on certain days of the year. IIRC there are also a few other unused unit and building graphics from Conquests scenarios that were ultimately left out of the final release.