Dropped ring in defrost vent on dash driver's side?

Okay, in that system it would go from the defrost duct down into the HVAC case through the defrost vents. Then into the mode door that selects where the air goes (number 1). If defrost was on, then it would likely have gone through the door into one of the two duct systems that are part of the case. Those split the driver and passenger side. Where the ring went is hard to say because it depends on what it hit on the way through and how the truck was oriented as it fell.

That depends on the way you got to it. That looks like the blend door and core. If possible, I would use a grabber tool to go in and grab the ring. Or make a hook to attach to the camera, however you would need to be sure of it holding the ring so something shaped like a circle with a narrow slot on top that would slip over the ring then rotate so it can't fall off during retrieval sort of like a circle fish hook but with the opening a bit tighter.

No real easy way as that area in the housing doesn't have any access. You would need to do the same work as changing the heater core which is basically stripping out the entire dash. With it straight down the hook method would work better.