I worked on Agent and there isn't a whole lot more to be seen other than what has already leaked.
"Alpine" was basically a demo of the combat and stealth mechanics while the "Desert" demo was more about ambience and sound design. Desert was much further along than Alpine with a relatively complete urban setting.
So what happened to Agent?
The short answer is that the clusterf*cks that were the development of RDR and L.A. Noire sucked all the resources out of Agent and once those titles had shipped GTA V was demanding everyone's resources.
My uncle's brother works at Rockstar and he says you're full of sh*t unless you prove it.